The book discusses the theoretical path to decoding the information gathered from observations of old stellar systems. It focuses on old stellar systems because these are the fossil record of galaxy formation and provide invaluable information ont he evolution of cosmic structures and the universe as a whole. The aim is to present results obtained in the past few years for theoretical developments in low mass star research and in advances in our knowledge of the evolution of old stellar systems. A particularly representative case is the recent discovery of multiple stellar populations in galactic globular clusters that represents one of the hottest topics in stellar and galactic astrophysics and is discussed in detail.Santi Cassisi has authored about 270 scientific papers, 150 of them in peer-reviewed journals, and the titleEvolution of Stars and Stellar Populations.Content:Chapter 1 Introduction (pages 1–18):Chapter 2 Low?Mass Star Physics (pages 19–81):Chapter 3 From the Main Sequence to the Tip of the Red Giant Branch (pages 83–161):Chapter 4 Horizontal Branch and Red Clump (pages 163–230):Chapter 5 Asymptotic Giant Branch (pages 231–274):Chapter 6 White Dwarf Cooling Sequences (pages 275–304):Chapter 7 Resolved Old Stellar Populations in the Galaxy (pages 305–398):Chapter 8 Resolved Composite Systems (pages 399–409):Chapter 9 Unresolved Old Systems (pages 411–480):