Context .- Thyroid carcinoma is the most common malignant tumor of endocrine organs, yet it only accounts for approximately 1% of all cancers in the United States with more than 35 000 new cases diagnosed each year and more than 450 000 people living with this disease. While most tumors can be diagnosed without much difficulty, a few tumor types, especially tumors with follicular pattern, sometimes pose a diagnostic challenge. Objective .- To discuss morphologic, immunohistochemical, and molecular features of thyroid tumors. We also explore the clinicopathologic features of papillary microcarcinoma and medullary microcarcinoma and how the latter is related and differentiated from C-cell hyperplasia. Finally with the ever-growing list of organ systems involved in immunoglobulin (Ig) G4-related diseases, we discuss the still not completely explored IgG-4-related thyroid disease. Data Sources .- Data were obtained from review of the pertinent peer-reviewed literature and institutional experience. Conclusions .- Histomorphologic evaluation still remains the gold standard for diagnosis in most cases of thyroid diseases. The application of ancillary studies such as immunohistochemistry and molecular diagnosis, including next-generation sequencing, is becoming more common.