diff --git a/dotnet/SafelyYou/Config/HeartBeatHistoryConfig.cs b/dotnet/SafelyYou/Config/HeartBeatHistoryConfig.cs
new file mode 100644
index 0000000..88fd548
--- /dev/null
+++ b/dotnet/SafelyYou/Config/HeartBeatHistoryConfig.cs
@@ -0,0 +1,6 @@
+namespace SafelyYou.Config;
+
+public class HeartBeatHistoryConfig
+{
+    public TimeSpan Window { get; set; }
+}
\ No newline at end of file
diff --git a/dotnet/SafelyYou/Devices.cs b/dotnet/SafelyYou/Devices.cs
new file mode 100644
index 0000000..ff69c49
--- /dev/null
+++ b/dotnet/SafelyYou/Devices.cs
@@ -0,0 +1,99 @@
+using System.Collections.Concurrent;
+using System.Runtime.InteropServices;
+using SafelyYou.Config;
+
+namespace SafelyYou;
+
+public readonly record struct DeviceId(string Value);
+
+
+public sealed class KnownDevices
+{
+    private readonly HashSet<DeviceId> _knownDevices;
+
+    public KnownDevices(HashSet<DeviceId> knownDevices) => _knownDevices = knownDevices;
+
+    public bool Contains(DeviceId deviceId) => _knownDevices.Contains(deviceId);
+}
+
+public readonly record struct HeartBeatRange(DateTimeOffset Start, DateTimeOffset? End);
+
+public sealed class DeviceHeartBeatHistory
+{
+    private readonly HeartBeatHistoryConfig _config;
+
+    public enum AddResult { Added, OutOfSequence }
+
+    public DeviceHeartBeatHistory(HeartBeatHistoryConfig config) => _config = config;
+
+    private readonly PriorityQueue<HeartBeatRange, DateTimeOffset> _histories = 
+        // lowest priority is first, but this allows the highest date to be dequeued first
+        new PriorityQueue<HeartBeatRange, DateTimeOffset>(new InverseDateTimeComparer());
+
+    private sealed class InverseDateTimeComparer : IComparer<DateTimeOffset>
+    {
+        public int Compare(DateTimeOffset x, DateTimeOffset y) => -(x.CompareTo(y));
+    }
+
+    /// <summary>Returns Added, needed to be used as an expression</summary>
+    private static AddResult Replace(PriorityQueue<HeartBeatRange, DateTimeOffset> histories, HeartBeatRange updated)
+    {
+        histories.DequeueEnqueue(updated, updated.Start);
+        return AddResult.Added;
+    }
+
+    /// <summary>Returns Added, needed to be used as an expression</summary>
+    private static AddResult EnqueueNew(PriorityQueue<HeartBeatRange, DateTimeOffset> histories, DateTimeOffset sentAt)
+    {
+        histories.Enqueue(new HeartBeatRange(sentAt, null), sentAt);
+        return AddResult.Added;
+    }
+    
+    public AddResult Add(DateTimeOffset sentAt)
+    {
+        // check the latest first, assuming things are in order
+        // if it's "in the past" and doesn't make sense, return an error
+        
+        // histories that are adjacent are combined, if there's more than a one minute gap (what about a margin of error?)
+        // then create a new entry. The margin is coming from the config. I've set it to 1 second.
+        
+        if (!_histories.TryPeek(out var last, out _))
+            return EnqueueNew(_histories, sentAt);
+
+        // last registered heartbeat is End if set, otherwise Start
+        return (sentAt, last.Start, last.End) switch
+        {
+            var (sent, start, _) when sent < start => AddResult.OutOfSequence,
+            (var sent, _, { } end) when sent < end => AddResult.OutOfSequence,
+            // no End yet: extend if within Window of Start
+            (var sent, var start, null) when sent <= start + _config.Window => Replace(_histories, last with { End = sentAt }),
+            // End set: extend if within Window of End
+            (var sent, _, { } end) when sent <= end + _config.Window => Replace(_histories, last with { End = sentAt }),
+            // gap larger than Window: start a new range
+            _ => EnqueueNew(_histories, sentAt),
+        };
+    }
+}
+
+public sealed class HeartBeatHistories
+{
+    private readonly HeartBeatHistoryConfig _config;
+    private readonly ConcurrentDictionary<DeviceId, DeviceHeartBeatHistory> _histories = new();
+
+    public HeartBeatHistories(HeartBeatHistoryConfig config) => _config = config;
+
+    public DeviceHeartBeatHistory.AddResult Add(DeviceId deviceId, DateTimeOffset sentAt)
+    {
+        if (!_histories.TryGetValue(deviceId, out var history))
+        {
+            history = new(_config);
+            _histories[deviceId] = history;
+        }
+
+        return history.Add(sentAt);
+    }
+}
+
+
+
+
diff --git a/dotnet/SafelyYou/Program.cs b/dotnet/SafelyYou/Program.cs
index ee9d65d..7baefc0 100644
--- a/dotnet/SafelyYou/Program.cs
+++ b/dotnet/SafelyYou/Program.cs
@@ -1,9 +1,28 @@
+using Microsoft.AspNetCore.Http.HttpResults;
+using SafelyYou;
+using SafelyYou.Api;
+using SafelyYou.Config;
+
 var builder = WebApplication.CreateBuilder(args);
 
 // Add services to the container.
 // Learn more about configuring OpenAPI at https://aka.ms/aspnet/openapi
 builder.Services.AddOpenApi();
 
+builder.Services.AddSingleton<KnownDevices>(_ =>
+{
+    var deviceIds = File.ReadAllLines("/devices.csv").Skip(1).Select(id => new DeviceId(id)).ToHashSet();
+    return new KnownDevices(deviceIds);
+});
+
+builder.Services.AddSingleton<HeartBeatHistoryConfig>(_ =>
+{
+    var millis = builder.Configuration.GetSection("HeartBeatHistory").GetValue<int>("WindowMillis");
+    return new HeartBeatHistoryConfig() { Window = TimeSpan.FromMilliseconds(millis) };
+});
+
+builder.Services.AddSingleton<HeartBeatHistories>();
+
 var app = builder.Build();
 
 // Configure the HTTP request pipeline.
@@ -14,6 +33,31 @@ if (app.Environment.IsDevelopment())
 
 app.UseHttpsRedirection();
 
+// heartbeat history
+// upload stats
+
+
+app.MapPost("/devices/{deviceId}/heartbeat", (string deviceId, HeartbeatRequest request,
+    KnownDevices knownDevices, HeartBeatHistories histories) =>
+{
+    var device = new DeviceId(deviceId);
+    if (!knownDevices.Contains(device))
+    {
+        return Results.NotFound(new NotFoundResponse($"Device ID {deviceId} not found"));
+    }
+
+    var addResult = histories.Add(device, request.SentAt);
+
+    return addResult switch
+    {
+        DeviceHeartBeatHistory.AddResult.Added => Results.Ok(),
+        DeviceHeartBeatHistory.AddResult.OutOfSequence => Results.InternalServerError(
+            new ErrorResponse("Attempted to add a timestamp that was out of sequence or invalid.")),
+        _ => throw new ArgumentOutOfRangeException()
+    };
+});
+
+
 var summaries = new[]
 {
     "Freezing", "Bracing", "Chilly", "Cool", "Mild", "Warm", "Balmy", "Hot", "Sweltering", "Scorching"
diff --git a/dotnet/SafelyYou/SafelyYou.csproj b/dotnet/SafelyYou/SafelyYou.csproj
index 8f692f0..4338008 100644
--- a/dotnet/SafelyYou/SafelyYou.csproj
+++ b/dotnet/SafelyYou/SafelyYou.csproj
@@ -10,4 +10,15 @@
     <PackageReference Include="Microsoft.AspNetCore.OpenApi" Version="10.0.7" />
   </ItemGroup>
 
+  <ItemGroup>
+    <Folder Include="Api\" />
+  </ItemGroup>
+
+  <ItemGroup>
+    <None Remove="devices.csv" />
+    <Content Include="devices.csv">
+      <CopyToOutputDirectory>PreserveNewest</CopyToOutputDirectory>
+    </Content>
+  </ItemGroup>
+
 </Project>
diff --git a/dotnet/SafelyYou/appsettings.json b/dotnet/SafelyYou/appsettings.json
index 10f68b8..6b45987 100644
--- a/dotnet/SafelyYou/appsettings.json
+++ b/dotnet/SafelyYou/appsettings.json
@@ -1,4 +1,7 @@
 {
+  "HeartBeatHistory" : {
+    "WindowMillis" : 1000 
+  },
   "Logging": {
     "LogLevel": {
       "Default": "Information",
diff --git a/dotnet/SafelyYou/devices.csv b/dotnet/SafelyYou/devices.csv
new file mode 100644
index 0000000..5ed9930
--- /dev/null
+++ b/dotnet/SafelyYou/devices.csv
@@ -0,0 +1,6 @@
+device_id
+60-6b-44-84-dc-64
+b4-45-52-a2-f1-3c
+26-9a-66-01-33-83
+18-b8-87-e7-1f-06
+38-4e-73-e0-33-59
diff --git a/dotnet/SafelyYou/openapi.json b/dotnet/SafelyYou/openapi.json
new file mode 100644
index 0000000..28882e2
--- /dev/null
+++ b/dotnet/SafelyYou/openapi.json
@@ -0,0 +1,193 @@
+{
+  "openapi": "3.0.3",
+  "info": {
+    "title": "Fleet Management Simple Metrics Server",
+    "description": "Simple and correct implementation of the Fleet Management Metrics Coding Assessment",
+    "contact": {
+      "name": "API Support",
+      "url": "http://www.example.com/support",
+      "email": "support@example.com"
+    },
+    "license": {
+      "name": "None",
+      "url": "https://safely-you.com"
+    },
+    "version": "1.0.0"
+  },
+  "servers": [
+    {
+      "url": "http://127.0.0.1:6733/api/v1",
+      "description": "local server"
+    }
+  ],
+  "paths": {
+    "/devices/{device_id}/heartbeat": {
+      "post": {
+        "description": "Register a heartbeat from a device",
+        "parameters": [
+          {
+            "$ref": "#/components/parameters/DeviceIDPathParam"
+          }
+        ],
+        "requestBody": {
+          "content": {
+            "application/json": {
+              "schema": {
+                "title": "HeartbeatRequest",
+                "required": ["sent_at"],
+                "properties": {
+                  "sent_at": {
+                    "type": "string",
+                    "format": "date-time"
+                  }
+                }
+              }
+            }
+          }
+        },
+        "responses": {
+          "204": {
+            "description": "the request was completed successfully"
+          },
+          "404": {
+            "$ref": "#/components/responses/NotFound"
+          },
+          "500": {
+            "$ref": "#/components/responses/Error"
+          }
+        }
+      }
+    },
+    "/devices/{device_id}/stats": {
+      "post": {
+        "description": "Add per device statistics",
+        "parameters": [
+          {
+            "$ref": "#/components/parameters/DeviceIDPathParam"
+          }
+        ],
+        "requestBody": {
+          "content": {
+            "application/json": {
+              "schema": {
+                "title": "UploadStatsRequest",
+                "required": ["sent_at", "upload_time"],
+                "properties": {
+                  "sent_at": {
+                    "type": "string",
+                    "format": "date-time"
+                  },
+                  "upload_time": {
+                    "description": "the number of nanoseconds it took to upload a video",
+                    "type": "integer"
+                  }
+                }
+              }
+            }
+          }
+        },
+        "responses": {
+          "204": {
+            "description": "the request was completed successfully"
+          },
+          "404": {
+            "$ref": "#/components/responses/NotFound"
+          },
+          "500": {
+            "$ref": "#/components/responses/Error"
+          }
+        }
+      },
+      "get": {
+        "description": "Return device stats",
+        "parameters": [
+          {
+            "$ref": "#/components/parameters/DeviceIDPathParam"
+          }
+        ],
+        "responses": {
+          "200": {
+            "description": "Device statistics",
+            "content": {
+              "application/json": {
+                "schema": {
+                  "title": "GetDeviceStatsResponse",
+                  "required": ["avg_upload_time", "uptime"],
+                  "properties": {
+                    "avg_upload_time": {
+                      "description": "returned as a time duration string. Eg: 5m10s",
+                      "type": "string"
+                    },
+                    "uptime": {
+                      "description": "Uptime as a percentage. eg: 98.999",
+                      "type": "number",
+                      "format": "double"
+                    }
+                  }
+                }
+              }
+            }
+          },
+          "204": {
+            "description": "the request was completed successfully"
+          },
+          "404": {
+            "$ref": "#/components/responses/NotFound"
+          },
+          "500": {
+            "$ref": "#/components/responses/Error"
+          }
+        }
+      }
+    }
+  },
+  "components": {
+    "parameters": {
+      "DeviceIDPathParam": {
+        "name": "device_id",
+        "in": "path",
+        "description": "ID of a device to register heartbeat with",
+        "required": true,
+        "schema": {
+          "type": "string"
+        }
+      }
+    },
+    "responses": {
+      "NotFound": {
+        "description": "Device not found",
+        "content": {
+          "application/json": {
+            "schema": {
+              "title": "NotFoundResponse",
+              "type": "object",
+              "required": ["msg"],
+              "properties": {
+                "msg": {
+                  "type": "string"
+                }
+              }
+            }
+          }
+        }
+      },
+      "Error": {
+        "description": "Server Error",
+        "content": {
+          "application/json": {
+            "schema": {
+              "title": "ErrorResponse",
+              "type": "object",
+              "required": ["msg"],
+              "properties": {
+                "msg": {
+                  "type": "string"
+                }
+              }
+            }
+          }
+        }
+      }
+    }
+  }
+}