Interactions between light and matter impart distinctive signatures on the polarization state of the radiation field. Astronomical polarimetry, the measurement of polarized light from astrophysical sources, enables these detailed processes to be probed and provides insights beyond that which can be measured through spectroscopy and photometry alone. For these reasons, polarimetry has increasingly played an important role in tool chest employed by astronomers. Advances in polarimetric techniques have enabled its use in studying questions ranging from the role of magnetic fields in regulating the process of star formation to the origin of the big bang. This book will present the theory, instrumental techniques, and applicable astrophysics and present them in a coherent narrative. The narrative will be used to illuminate how polarization arises in astrophysical settings as well as convey how instrumental contributions to the observations arise, can be mitigated in the instrument design, and controlled in data reduction. Example architectures of polarimeters from real-world implementations will be utilized to illustrate the instrumental techniques, and the book will include associated python libraries for calculating useful quantities.