GP Workbench is an open-source general-purpose geophysical data processing software package written primarily for ground penetrating radar (GPR) data. It also includes support for several USGS prototype electromagnetic instruments such as the VETEM and ALLTEM. The two main programs in the package are GP Workbench and GP Wave Utilities. GP Workbench has routines for filtering, gridding, and migrating GPR data; as well as an inversion routine for characterizing UXO (unexploded ordinance) using ALLTEM data. GP Workbench provides two-dimensional (section view) and three-dimensional (plan view or time slice view) processing for GPR data. GP Workbench can produce high-quality graphics for reports when Surfer 8 or higher (Golden Software) is installed. GP Wave Utilities provides a wide range of processing algorithms for single waveforms, such as filtering, correlation, deconvolution, and calculating GPR waveforms. GP Wave Utilities is used primarily for calibrating radar systems and processing individual traces. Both programs also contain research features related to the calibration of GPR systems and calculating subsurface waveforms. The software is written to run on the Windows operating systems. GP Workbench can import GPR data file formats used by major commercial instrument manufacturers including Sensors and Software, GSSI, and Mala. The GP Workbench native file format is SU (Seismic Unix), and subsequently, files generated by GP Workbench can be read by Seismic Unix as well as many other data processing packages.