The tumor environment is a dynamic network that includes cancer cells, immune cells, fibroblasts, endothelial cells, extracellular matrix, cytokines and receptors. The aim of this book is to summarize the role of these components, especially immune cells, in tumor suppression and/or progression and describe in detail why tumor cells can survive and spread in spite of the antitumor response of immune cells. Since immunotherapy is an attractive approach to cancer therapy, this book also provides information on the two main strategies: monoclonal antibodies and adaptive T cell immunotherapy, with a focus on recent human clinical trials.