In mathematics, technical difficulties can spark groundbreaking ideas. This book explores one such challenge: a problem that arose in the formative years of algebraic number theory and played a major role in the early development of the field. When nineteenth-century mathematicians set out to generalize E. E. Kummer's theory of ideal divisors in cyclotomic fields, they discovered that the existence of ""common inessential discriminant divisors"" blocked the obvious path. Through extensively annotated translations of key papers, this book traces how Richard Dedekind, Leopold Kronecker, and Kurt Hensel approached these divisors, using them to justify the need for entirely new mathematical ideas and to demonstrate their power. Mathematicians interested in algebraic number theory will enjoy seeing what the field, which is still evolving today, looked like in its very early days. Historians of mathematics will find interesting questions for further study. Engaging and carefully researched, Common Inessential Discriminant Divisors is both a historical study and an invitation to experience mathematics as it was first discovered.