This book will explore the many biological and physical aspects of the tumor microenvironment. The book will include chapters on the various cells that influence tumor growth and function as well as the cell-associated and soluble proteins that can promote invasion and metastasis. Several chapters will describe endothelial cells and pericytes that form tumor vasculature. Insights on the role of progenitor and stem cells will be included. The contribution of the supporting stroma will be addressed in addition to cell-cell signaling and cell-matrix interactions. Additional chapters will describe the influence of infiltrating cells of the immune system on tumor growth.