Book excerpt: This book presents detailed information regarding the innate and adaptive cellular diversity in tumor microenvironment. Tumor microenvironment refers to a dynamic niche outlined by the interaction of diverse cell types such as tumor cells and stromal cells, their soluble variants and different physicochemical circumstances. Current studies have distinguished myelomonocytic cells as important factors in the regulation of tumor microenvironment and thus, tumor advancement in numerous kinds of cancers. Keeping these discoveries in mind, this book has been compiled in an effort to bring forth an extensive description on the varieties of tumor microenvironment in different cancers and shed light on the critical role of myelomonocytic cells in stemming this niche for monitoring cancer progression. This book will provide a better understanding of mechanisms of myelomonocytic cells in regulating cancer progression which may lead to new developments in cancer therapeutics.