This book provides a simple and well-structured course followed by an innovative collection of exercises and solutions that will enrich a wide range of courses as part of the undergraduate physics curriculum. It will also be useful for first-year graduate students who are preparing for their qualifying exams. The book is divided into four main themes at the boundary of classical and modern physics: atomic physics, matter-radiation interaction, blackbody radiation, and thermodynamics. Each chapter starts with a thorough and well-illustrated review of the core material, followed by plenty of original exercises that progress in difficulty, replete with clear, step-by-step solutions. This book will be invaluable for undergraduate course instructors who are looking for a source of original exercises to enhance their classes, while students that want to hone their skills will encounter challenging and stimulating problems.