Powersort is an adaptive sorting algorithm designed to optimally exploit existing order in the input data with minimal overhead. Since version 3.11, Powersort is the default list-sorting algorithm in CPython and is also used in NumPy, PyPy, AssemblyScript, and Apple's WebKit. Powersort belongs to the family of merge sort algorithms. More specifically, Powersort builds on Timsort; it is a drop-in replacement for Timsort's suboptimal heuristic merge policy. Unlike the latter, it is derived from first principles and offers strong performance guarantees.