This book studies the relationship between number theory in algebraic number fields and algebraic function fields. Because function fields are a bit different from number fields, even the experienced number theorist will learn from this book. Algebraic geometers will like the book, since the geometry of curves over an algebraically closed field is both pretty and elementary. Michael Rosen is the author of the successful book "A Classical Introduction to Modern Number Theory." He is the recipient of the 1999 Chauvenet Prize for his article "Niels Hendrik Abel and Equations of the Fifth Degree."