ASP.NET is Microsoft’s framework for building web applications and services on the .NET platform. It began as part of the original .NET Framework, shipping in 2002 with a programming model called Web Forms that brought the event-driven, control-based style familiar from Windows desktop development to the web. Web Forms abstracted HTTP behind server-side controls and a page lifecycle, with view state used to preserve UI data across requests, making it approachable for developers coming from rich-client backgrounds.
As web development shifted toward cleaner separation of concerns and more direct control over the generated HTML, Microsoft introduced ASP.NET MVC, which applied the model-view-controller pattern and gave developers explicit routing, testable controllers, and templated views without the heavy abstractions of Web Forms. Alongside it, ASP.NET Web API provided a focused way to build HTTP and REST services. For a decade these frameworks ran on the Windows-only .NET Framework and were hosted primarily through Internet Information Services.
The most significant break came with ASP.NET Core, a ground-up rewrite first released in 2016. Microsoft re-architected the framework to be modular, lightweight, and cross-platform. The official documentation describes it as “an open-source and cross-platform framework for building modern cloud-based internet-connected applications, such as web apps, IoT apps, and mobile backends,” running on Windows, macOS, and Linux. ASP.NET Core unified the previously separate MVC and Web API models, introduced a composable middleware pipeline and built-in dependency injection, and could run behind its own high-performance Kestrel server rather than depending on IIS.
ASP.NET Core also embraced open development. Its source lives in a public repository on GitHub, maintained under the .NET Foundation with an MIT license, and accepts community contributions, a sharp departure from the closed development of the early framework. The same codebase hosts newer programming models, including minimal APIs for terse service definitions and Blazor for building interactive UIs in C# rather than JavaScript.
The arc from Web Forms to ASP.NET Core mirrors the broader evolution of Microsoft’s developer platform: from a Windows-centric, proprietary stack toward an open-source, cross-platform one. ASP.NET remains one of the principal ways that C# and .NET developers build server-side web applications and APIs, and its modern, modular design positioned the platform to compete directly with frameworks from the Java and JavaScript worlds.