Optical microscopy and associated technologies advanced quickly after the introduction of the laser. The techniques have stimulated further development of optical imaging theory, including 3-dimensional microscopy imaging theory in spatial and frequency domains, the theory of imaging with ultrashort-pulse beams and aberration theory for high-numerical-aperture objectives. This book introduces these new theories in terms of modern optical microscopy. It consists of seven chapters including an introduction. The chapters are organized to minimize cross-referencing. Comparisons with classical imaging theory are made when the new imaging theory is introduced. The book is intended for senior undergraduate students in courses on optoelectronics, optical engineering, photonics, biophotonics and applied physics, after they have completed modern optics or a similar subject. It is also a reference for other scientists interested in the field.