The thyroid gland is centrally important in metabolic homeostasis, growth, and development. Defects in any of the multiple steps required for normal thyroid development and thyroid hormone synthesis have been shown to result in thyroid-dependent pathology in a variety of animal models of disease and in humans. However, there are many unanswered questions regarding the cellular and molecular mechanisms leading to thyrocyte abnormalities. A lack of knowledge regarding thyroid stem/progenitor cell differentiation and proliferation has further hampered attempts to gain more insight into thyroid-associated disease mechanisms. In this chapter, we present a culture system that has successfully generated thyrocyte-like cells from undifferentiated embryonic stem cells in vitro. Access to specific early-developing cell populations, such as the thyroid stem cells in the embryonic stem cell system, provides an experimental approach to characterizing key molecular events involved in thyrocyte commitment.