Immune checkpoints are among the maintenance regulators of immune responses. In this book, all inhibitory checkpoint molecules in the immune system will explain. In the first part, immune checkpoint molecules will be categorized and described in terms of structure, mechanisms, cellular expression, and genetics in detail. The next part will describe the role of these receptors in tumors and the therapeutic approach in cancers. Eventually, the role of immune checkpoints in other clinical situations, such as transplantation, autoimmunity, and other diseases will be discussed. It is worth mentioning many sources have described inhibitory checkpoint molecules in cancers since there are poor sources that collected all inhibitory checkpoint molecules. This book tries to categorize and introduce all these molecules based on their application and importance in health and disease. It aims to provide general information about immune checkpoints that will be useful for scientists and clinicians.