The GNU Radio Companion (GRC) is a tool that allows users of all levels to design and modify high-rate signal processing applications. It is the GUI frontend for GNU Radio application designers.

GNU Radio is centered around the concept of the Block, which is a node in a signal processing flow graph. Since GNU Radio itself comes with a lot of useful blocks already, one can start developing real-time and hardware-interfacing or simulation flow graphs right away, using GRC as the tool to connect and parameterize these blocks.

GRC itself generates a program in a target language (Python or C++), which contains all the signal flow setup as defined visually.