Active Server Pages (ASP) also known as Classic ASP or ASP Classic, was Microsoft's first server-side script engine for dynamically-generated web pages. Initially released as an add-on to Internet Information Services (IIS) via the Windows NT 4.0 Option Pack, it was subsequently included as a free component of Windows Server since the initial release of Windows 2000 Server. It has now been superseded by ASP.NET.

Developing functionality in ASP websites is enabled by the active scripting engine's support of the Component Object Model (COM), with each object providing a related group of frequently-used functions and data attributes.

Web pages with the .asp file extension use ASP. Pages with the .aspx extension are ASP.NET based on Microsoft's .NET Framework and compiled which makes them faster and more robust than server-side scripting in ASP which is interpreted at run-time. Many ASP.NET pages still include some ASP scripting. Such marked differences between ASP and ASP.NET have led to the terms Classic ASP or ASP Classic being used, which also implies some nostalgia for the simpler platform.

Most ASP pages are written in VBScript, but any other Active Scripting engine can be selected instead by using the @Language directive or the <script language="language" runat="server"> syntax.

Active Server Pages provides all of the functionality of CGI applications in an easier-to-use and more robust environment. ASP is an easier way for your server to access information in a form not readable by the client (such as an SQL database) and then act as a gateway between the two to produce information that the client can view and use. With CGI, the server creates as many processes as the number of client requests received. The more concurrent requests there are, the more concurrent processes created by the server. However, creating a process for every request is time-consuming and requires large amounts of server memory. In addition, this can restrict the resources available for sharing from the server application itself, slowing down performance and increasing wait times on the Web.

Active Server Pages runs in the same process as the Web Server, handling client requests faster and more efficiently. It is much easier to develop dynamic content and Web applications with ASP.