Neuroendocrine tumors may develop throughout the physical body with the bulk being found within the alimentary canal and bronchopulmonary system. Neuroendocrine tumors are classified consistent with the grade of biological aggressiveness (G1–G3) and therefore the extent of differentiation (well-differentiated/poorly-differentiated). The well-differentiated neoplasms comprise typical (G1) and atypical (G2) carcinoids. Large cell neuroendocrine carcinomas also as small cell carcinomas (G3) are poorly-differentiated. The identification and differentiation of atypical from typical carcinoids or large cell neuroendocrine carcinomas and little cell carcinomas is important for treatment options and prognosis. Chromosomal aberrations are more likely seen in neuroendocrine carcinomas. Such structural changes of chromosomal sections are more common in high-grade neuroendocrine carcinomas and atypical carcinoids than typical well-differentiated ones. Lung NETs are characterized by strikingly heterogeneous pathological features and clinical behavior. At one end of the spectrum are the so-called typical carcinoids, which are well-differentiated, low-grade, slowly growing neoplasms that seldom metastasize to extrathoracic structures. At the opposite end of the spectrum are the poorly differentiated and high-grade neuroendocrine carcinomas, as typified by small cell carcinoma , which behaves aggressively, with rapid tumor growth and early distant dissemination.