This book is an introduction to the theory of Lie groups and Lie algebras, with emphasis on the theory of semi simple Lie algebras and representations theory.It is sure to become a standard textbook for graduate students in mathematics and physics with little or no prior exposure to Lie theory. To keep the prerequisites to a minimum, the author restricts attention to matrix Lie groups and Lie algebras.This approach keeps the discussion concrete, allows the reader to get to the heart of the subject quickly, and covers all the most interesting examples.