Atomistix ToolKit (ATK) is a commercial software for atomic-scale modeling and simulation of nanosystems. The software was originally developed by Atomistix A/S, and was later acquired by QuantumWise A/S following the Atomistix bankruptcy. Atomistix ToolKit is a further development of TranSIESTA-C, which in turn in based on the technology, models, and algorithms developed in the academic codes TranSIESTA and McDCal, employing localized basis sets as developed in SIESTA.