This book is an English translation of Die Lehre von der Bildentstehung im Mikroskop von Ernst Abbe, the only published detailed account of Abbe's theory of image formation in the microscope. The original German edition, written and published by Otto Lummer and Fritz Reiche in 1910, was an expanded version of the lectures given by Abbe in 1887. The book presents an introduction to geometrical optics, discusses image formation theory based on optical diffraction, and deals with optical images of several kinds of objects being illuminated on and off axis. It also introduces coherent imaging as two back-to-back diffraction processes and discusses the resolution limit of an imaging system. The book concludes with a discussion of the effect of artificial blocking of certain diffraction orders on the final image. This translation, which includes annotations and other added material, can serve as a self-study book for readers who wish to learn optics and optical image formation. It is also a tribute to the original authors' scientific achievements and devotion to the teaching and dissemination of precious knowledge.