pThis textbook provides an introduction to gravitational lensing which has become an invaluable tool in modern astrophysics with applications that range from finding planets orbiting distant stars to understanding how dark matter and dark energy conspired to form the cosmic structures we see today.ampnbspPrinciples of Gravitational Lensingampnbspbegins with Einsteins prediction that gravity bends light and shows how that fundamental idea has spawned a rich field of study over the past century.ppThe gravitational deflection of light was first detected by Eddington during a solar eclipse in May 1919 launching Einstein and his theory of relativity into public view. Yet the possibility of using the phenomenon to unlock mysteries of the Universe seemed remote given the technology of the day. Theoretical work was carried out sporadically over the next six decades but only with the discovery of the system Q0957561 in 1979 was gravitational lensing transformed from a curiosity of general relativity into a practical observational tool.ppThis book describes how the three subfields known as strong lensing weak lensing and microlensing have grown independently but become increasingly intertwined. Drawing on their research experienceampnbspCongdonampnbspand Keeton begin with the basic physics of light bending then present the mathematical foundations of gravitational lensing building up to current research topics in a clear and systematic way. Relevant background material from physics and mathematics is included making the book selfcontained.ppThe derivations and explanations are supplemented by exercises designed to help students master the theoretical concepts as well as the methods that drive current research.