SpecTcl is a framework for data analysis of nuclear physics event data. It is delivered both as a canned program and as a C++ class framework. It is run-time extensible via the Tcl/Tk scripting language. The class framework allows the program to be easily extended, and class wrappers for Tcl/Tk make it trivial to add to the SpecTcl command set as well as to expose Tcl/Tk variables to user code.