Gdstk (GDSII Tool Kit) is a C++ library for creation and manipulation of GDSII and OASIS files. It is also available as a Python module meant to be a successor to Gdspy. Key features for the creation of complex CAD layouts are included: boolean operations on polygons (AND, OR, NOT, XOR) based on clipping algorithm; polygon offset (inward and outward rescaling of polygons); efficient point-in-polygon solutions for large array sets.