This work covers the pathophysiology of cancer, exploring the difficulty of optimal treatment due to the complexity and diversity of cancer types. The search for distinctive molecular biology characteristics of tumor cells is especially relevant in the identification of overexpressed receptors and proteins that can be used as a target for cancer treatment.We highlight the main therapeutic modalities, particularly conventional systemic chemotherapy, addressing its mechanisms of action, therapeutic classes and even the toxic effects. We also describe the main tumor markers, their importance in the diagnosis and treatment of cancer, and the specificity of tumor cells.   The first chapters serve as an introduction to the central topic of this book, targeted therapy.