Pulmonary neuroendocrine tumors are frequently encountered as small biopsy and resection specimens. Although the World Health Organization (WHO) classification is used as a diagnostic framework globally and in daily practice, it is not without controversy and practical difficulties. The WHO criteria for pulmonary neuroendocrine tumors are reviewed with the recognition that the definitions are based on resected specimens, which are often not the usual clinical circumstances. A practical approach is emphasized, and the differential diagnosis is discussed with particular attention to the context in which diagnostic difficulties are most frequently encountered, such as small biopsies and frozen sections.