TBTK: A quantum mechanics software development kit. TBTK is a software development kit for quantum mechanical calculations and is designed to enable the development of applications that investigate problems formulated on second-quantized form. It also enables method developers to create solvers for tight-binding, DFT, DMFT, quantum transport, etc., that can be easily integrated with each other. Both through the development of completely new solvers, as well as front and back ends to already well established packages. TBTK provides data structures tailored for second-quantization that will encourage reusability and enable scalability for quantum mechanical calculations.