Biomechanical ToolKit (BTK) is a cross-platform library for biomechanical analysis. It can read and write a large variety of file formats used in biomechanics, and can modify them. All these operations can be done with the C++ API or with the wrappers included (Python, Octave, and Matlab). The goal of this project is to help the community share data without the restriction of the file format or the biomecanical model provided by the manufacturer of the acquisition system.