Solar and stellar atmospheres, magnetospheres of planets or coronae of magnetars are in the state of plasma. Even dust particles in the cosmic space can be charged, thus forming a plasma. Therefore, plasma astrophysics is a vast field which is impossible to cover in one book, especially owing to its fast development given by new observations made by ground-based and satellite instruments. This textbook is intended as an introduction to plasma astrophysics and plasma physics of solar flares. It is addressed to graduate and doctoral students. It can be of interest of students in theoretical physics, space plasma physics and laboratory plasma physics and it is based on author's experience in research studies of solar flares and teaching courses on plasma astrophysics at Faculty of Mathematics and Physics, Charles University in Prague.