After the pioneering work of Schur, little progress was made for half a century on projective representations, despite considerable activity on the related topic of linear representations. However, in the last twenty years, important new advances have spurred further research. This book develops both the early theory of Schur and then describes the key advances that the subject has seen since then. In particular, the theory of Q-functions and skew Q-functions is extensively covered which is central to the development of the subject.