Slicer is easily extendable due to its modular structure. Each module automatically gets its own panel in the Slicer GUI, which the developer then fleshes out to create its own user interface. Since Slicer is built on VTK, the Visualization Toolkit, modules have access to a wide range of visualization capabilities.

The source code of Slicer can be divided into two main parts: Tcl/Tk modules and C++ code that extends VTK. Modules are written directly in Tcl/Tk. C++ code is wrapped in Tcl when VTK is compiled.