Optical lithography is one of the key technologies in the fabrication of microchips. This book gives an account of modern lithography during the first half of the 2020s, when high-NA extreme-ultraviolet lithography (EUVL) was being introduced. Central to the book is a comprehensive description of the effects that impact the quality of the optical image inside the resist layer, a story that has become more complicated with every new lithography tool generation. This discussion builds gradually so that anyone with a graduate-type background in physics, chemistry, or mathematics can follow it from the start to the many subtleties of image formation in high-NA EUVL. It uses mathematics when useful and is accompanied by examples and case studies, many of which were created specifically for this book. This book will be of value for people who are new to lithography as well as for the more experienced lithographer.