Myostatin, also known as growth and differentiation factor-8, is a member of the TGF-β family member and is mainly produced by skeletal muscle. Myostatin circulates in the serum in a latent form, whereas it acts almost exclusively on skeletal muscle. Chapter One of this book discusses the role of myostatin and related factors in muscle hypertrophy and atophy. Chapter Two concentrates on specific alterations of positive and negative regulators of muscle mass in patients with chronic kidney disease. Chapter Three examines the potential benefits of the myostatin gene polymorphisms in livestock animals. Chapter Four studies myostatin functions in fish.