MEF is stand for Managed Extensibility Framework and is a library for creating lightweight, extensible applications for web development. All the web developers are use MEF for discover and use extensions without required the configurations of system. In MEF developers are easily encapsulate code and removed the hard code dependency. MEF only not allows extensions to be reused within applications. A MEF component which is used for declaratively specifies both its dependencies and what capabilities it makes available. MEF is used for solve the problems Because MEF parts declaratively specify their capabilities all the capabilities are discoverable at runtime means that an application can make use of parts without either hard-coded references or fragile configuration files.