Primary cell cultures of avian pinealocytes and the mammalian suprachiasmatic nucleus (SCN), immortalized cell lines derived from the SCN (SCN2.2), and fibroblasts derived from mice and rats have been employed as in vitro models to study the cellular and molecular mechanisms underlying circadian biological clocks. This article compares and contrasts these model systems and describes methods for avian pinealocyte cultures, immortalized SCN2.2 cells, and mouse fibroblast culture. Each of these culture models has advantages and disadvantages. Avian pinealocytes are photoreceptive, contain a circadian pacemaker, and produce rhythms of an easily assayed endocrine output-melatonin. However, the molecular mechanisms underlying pinealocyte function are not understood. SCN2.2 cells express metabolic and molecular rhythms and can impose rhythmicity on cocultured cells as well as rat behavior when transplanted into the brain. Yet, the entrainment pathways are not experimentally established in these cells. Fibroblast cultures are simple to produce and express molecular clock gene rhythms, but they express neither physiological rhythmicity nor pacemaker properties. The relative merits of these culture systems, as well as their impact on understanding circadian organization in vivo, are also considered.