This is the first book on JAK inhibitors and its contents are part of the previously published book “Systemic Drugs in Dermatology”. The book begins with basic and applied immunology. The sections in the book help the reader to understand why and how to predict JAK usage. The varied approved and unapproved usages followed by monitoring protocols and side effects are covered in this book. This is the era of specific targeted therapy and JAK inhibitors, novel drugs, with their ability to target cytokine/s families with the advantage of oral usage and their cost, is a harbinger of a therapeutic revolution.