Most lesions of the thyroid are follicular. These range from changes in the follicles seen diffusely, as in hyperthyroidism associated with Graves disease, to those lesions that present as multiple or single nodules.The solitary nodules of the thyroid, which are encapsulated, can be classified as adenoma, carcinoma, or a follicular variant of papillary carcinoma. Its peculiar nuclear features define a follicular variant of papillary carcinoma, which allows for its diagnosis in both cytologic and histologic preparations. The diagnostic distinctions between follicular adenoma and carcinoma necessitate the examination and interpretation of well-prepared histologic sections. Various criteria for the diagnoses of these neoplasms and groups of lesions will be discussed in this article. The article will concentrate on that group of lesions which are truly follicular neoplasms: adenoma and carcinoma.