Neoplasia refers to the process of formation of neoplasm, an abnormal mass of tissue that continues to grow in an excessive and uncoordinated fashion even after the cessation of the triggering stimulus. Each year thousands of individuals die of cancer-related deaths despite the advancements in the understanding of tumor biology, diagnostic modalities, and therapeutic interventions. This chapter presents a brief overview of the classification, microscopy, molecular genetics, carcinogenesis, clinical aspects, and diagnosis of neoplasms.