For almost two decades, Sidney Coleman has been giving review lectures on frontier topics in theoretical high-energy physics at the International School of Subnuclear Physics held each year at Erice, Sicily. This volume is a collection of some of the best of these lectures. To this day they have few rivals for clarity of exposition and depth of insight. Although very popular when first published, many of the lectures have been difficult to obtain recently. Graduate students and professionals in high-energy physics will welcome this collection by a master of the field.