The Oskar Klein Memorial Lecture series has become a very successful tradition in Swedish physics since it started in 1988. Theoretical high-energy physics dominates the subjects of the lectures, mirroring one of Klein's own main interests.This single volume is a compilation of the unique lectures previously produced in three separate volumes. The lectures are by world renowned experts in physics who have all contributed to the excitement of the field over the years. They continue to be of value to students and teachers alike.