An .fsproj file is an XML-based project file format primarily used within the Microsoft .NET ecosystem for F# programming language projects. It serves as the blueprint for building an F# application, library, or other component. Managed by development environments like Microsoft Visual Studio, Visual Studio Code, and JetBrains Rider, this file specifies crucial information required by the MSBuild build system. This includes a list of all source code files (.fs, .fsi) that comprise the project, references to external assemblies, NuGet packages, and other projects within the same solution. Furthermore, it defines build configurations (e.g., Debug, Release), target frameworks (such as .NET Core, .NET Framework, .NET 5+), output types (e.g., executable, library), and various compiler options. While typically managed by an Integrated Development Environment (IDE), .fsproj files are human-readable XML and can be manually edited, though this is less common for complex projects. They are fundamental for ensuring consistent and reproducible builds of F# applications across different development environments and continuous integration pipelines.