Papillary thyroid carcinomas are the most common endocrine cancer and are usually associated with good survival. However, some variants of papillary thyroid carcinomas may behave more aggressively than classic papillary thyroid carcinomas. The tall cell variant of papillary thyroid carcinoma is the most common aggressive variant of papillary thyroid carcinoma. The aggressive behavior has been ascribed to the histologic subtype and/or to the clinicopathologic features, an issue that remains controversial. The columnar variant of papillary thyroid carcinoma can be aggressive, particularly in older patients, with larger tumors showing a diffusely infiltrative growth pattern and extrathyroidal extension. A papillary thyroid carcinoma is designated as solid/trabecular variant when all or nearly all of a tumor not belonging to any of the other variants has a solid, trabecular, or nested (insular) appearance. This tumor must be distinguished from poorly differentiated thyroid carcinoma which has the same growth pattern but lacks nuclear features of papillary thyroid carcinoma and may show tumor necrosis and high mitotic activity. New to the fourth edition of the WHO Classification of Tumours of Endocrine Organs, the hobnail variant of papillary thyroid carcinoma is a moderately differentiated papillary thyroid carcinoma variant with aggressive clinical behavior and significant mortality. All of these variants are histologically unique and important to recognize due to their aggressive behavior.