Due to the advent of a new generation of detectors, X-ray polarimetry promises to join X-ray imaging, spectroscopy and timing as one of the main observational techniques in high energy astrophysics. This has renewed interest in the field, and indeed several polarimetric missions have recently been proposed. This volume provides a complete and up-to-date view of the subject for researchers in astrophysics. The contributors discuss the present status and perspectives of instruments, review current theoretical models, and examine future missions. As well as detailed papers, the book contains broad reviews that can be easily understood by astrophysicists new to the field.