Publisher Summary It is now recognized that cancer, in its simplest form, is a genetic disease or, more precisely, a disease of abnormal gene expression. Recent research efforts have revealed that different forms of cancer share common molecular mechanisms governing uncontrolled cellular proliferation, involving loss, mutation, or dysregulation of genes that positively and negatively regulate cell proliferation, migration, and differentiation (generally classified as proto-oncogenes and tumor suppressor genes). This chapter introduces basic and essential concepts related to neoplastic disease as a foundation for more detailed treatment of the molecular carcinogenesis of major cancer types. It provides an overview of cancer statistics and epidemiology, highlighting cancer types of importance to human health in the United States and worldwide, with a brief review of risk factors for the development of cancer. It discusses the classification of neoplasms, focusing on the general features of benign and malignant neoplasms, with an overview of nomenclature for human neoplasms, a description of preneoplastic conditions, and consideration of special subsets of neoplastic disease (cancers of childhood, hematopoietic neoplasms, and hereditary cancers). It also describes the distinguishing characteristics of benign and malignant neoplasms, with a focus on anaplasia and cellular differentiation, rate of growth, local invasiveness, and metastasis. The chapter further discusses the clinical aspects of neoplasia, giving an overview of cancer-associated pain, cancer cachexia, paraneoplastic syndromes, and methods for grading and staging of cancer.