The richness of nonlinear optics is infinite compared with the tiny organelle we call classical electromagnetism. If classical electromagnetism were an elementary particle, nonlinear optics would spill beyond the multiverse. If you are interested in learning about this fascinating field, the best teachers are those that can anticipate your questions and make clear those topics that are naturally confusing. What better teacher than a collection of students who have just recently mastered the material and who are fully aware of the struggles they had to overcome to get there? This informal textbook on nonlinear optics is a compilation of materials written by students who attended lectures by the award-winning teacher and researcher Regents Professor Mark G. Kuzyk of Washington State University. Material not normally discussed in standard textbooks covered here includes the introduction of second quantization and how it can be applied to Feynman-like diagrams for calculating nonlinear susceptibilities. This approach provides a pictorial representation of light-matter interactions that leads to a better and more intuitive understanding of phenomena such as difference frequency generation and nonlinear stimulated emission. Also included are unique problem sets that are not typically assigned in a course on nonlinear optics. This book may be a bit rough around the edges, and this may appear a bit quaint, but it gets the point across to novice students in a language they understand, and at a price that can't be beat.