This book provides an in-depth understanding of proteins and their interactions with each other. Proteins are vital in almost all biological procedures. The functions of proteins are synchronized through complex regulatory networks of transient protein-protein interactions (PPIs). To study PPIs, a broad range of methods have been developed over the past few decades. Several in vitro and in vivo attempts have been implemented to discover the mechanism of these pervasive interactions. Despite noteworthy developments in these investigational approaches, many issues exist such as false-positives/false-negatives, difficulty in obtaining crystal structures of proteins, etc. In order to overcome these challenges, various technical methods have been created, that are becoming more broadly used to examine PPIs. This book discusses various experimental approaches used to observe these interactions. It also looks at other important issues related to PPIs. The objective of this book is to bring together several researches accomplished by experts around the globe.