Most thyroid cancers are slow-growing, easily treatable tumors with an excellent prognosis after surgical resection and targeted medical therapy. Unfortunately, 10% to 15% of thyroid cancers exhibit aggressive behavior and do not follow an indolent course. Approximately one-third of patients with differentiated thyroid cancers will have tumor recurrences. Distant metastases are present in about 20% of patients with recurrent cancer. Approximately half of patients with distant metastases die within 5 years. The loss of the ability to concentrate radioiodine and produce thyroglobulin is a sign of dedifferentiation, which occurs in about 30% of patients with persistent or recurrent thyroid cancer. Dedifferentiation is associated with poorer responses to conventional therapy and difficulty monitoring tumor burden. Clinicians must identify tumors with more aggressive biology and treat them accordingly with more aggressive regimens. Part 1 of this two-part article describes in detail the distinct types of thyroid cancer, as well as risk factors, outcomes, and prognostic factors, with a focus on thyroid cancers of follicular cell origin. Part 2, which will appear in next month's issue, covers risk assessment and staging, findings that suggest the presence of aggressive tumors, recurrent/metastatic disease, and the value of treatment with chemotherapy and external-beam radiotherapy. Experimental treatments utilizing molecular targets, redifferentiation agents, and gene therapy are covered briefly as well.