.NET Framework was the predominant implementation of .NET technologiesCLI, until the release of [[.NET]]. Other implementations for parts of the framework exist. Although the runtime engine is described by an ECMA-ISO specification, other implementations of it may be encumbered by [[software patent|patent]] issues; ISO standards may include the disclaimer, "Attention is drawn to the possibility that some of the elements of this document may be the subject of patent rights. ISO shall not be held responsible for identifying any or all such patent rights."<ref>ISO 9001:2008, Foreword</ref> It is harder to develop alternatives to FCL, which is not described by an open standard and may be subject to copyright restrictions. Also, parts of FCL have Windows-specific functions and behavior, so implementation on non-Windows platforms can be problematic.
Some alternative implementations of parts of the framework are listed here.
