This text describes the microscopic physics operating in stars and demonstrates how stars respond from formation, through hydrogen-burning phases, up to the onset of helium burning. Intended for beginning graduate students and senior undergraduates with a solid background in physics, it illustrates the intricate interplay between the microscopic physical processes and the stars' macroscopic responses. The volume starts with the gravitationally contracting phase which carries the star from formation to the core hydrogen-burning main sequence, through the main sequence phase, through shell hydrogen-burning phases as a red giant, up to the onset of core helium burning. Particular emphasis is placed on describing the gravothermal responses of stars to nuclear transformations in the interior and energy loss from the surface, responses which express the very essence of stellar evolution. The volume is replete with many illustrations and detailed numerical solutions to prepare the reader to program and calculate evolutionary models.