Harmonica is a Python library for processing and modeling gravity and magnetic data. It includes common processing steps, like calculation of Bouguer and terrain corrections, reduction to the pole, upward continuation, equivalent layers, and more. There are forward modeling functions for basic geometric shapes, like spheres, prisms, polygonal prisms, and tesseroids. The inversion methods are implemented as classes with an interface inspired by scikit-learn (like Verde). Documentation: https://www.fatiando.org/harmonica Source code: https://github.com/fatiando/harmonica Part of the Fatiando a Terra project. Note: Authors are listed in alphabetical order by last name.