Although the chapters included in this book are all addressing protein interactions, we try to separate them into two parts according to their objectives. Chapters in part 1 mainly focus on the investigation of some specific protein-protein or protein-nucleic acid interactions, and try to elucidate the mechanism of specific cellular processes. Part 1 provides some insight of why and how to study protein interactions, and illustrates some approaches to modulate protein interactions. The second part is devoted to the development of various methods for the investigation of protein interactions, including computational modeling. Methods used to study protein interactions often evolve.