The term neoplasm, commonly used interchangeably with tumor, refers to an abnormal growth of cells. This growth may be benign without the potential for metastatic spread or malignant with a risk for metastatic spread to other anatomical sites. Both types of neoplasm might lead to morbidity and death, depending on factors such as the cell and site of origin, immunological responses, nutritional conditions, and anatomical extent. In this chapter, characteristic differences between different types of benign and malignant tumors are described. Further, we elaborate on their genesis and basis for survival and growth, as well as methods for clinical tumor tissue sampling and diagnosis.