Thyroid hormone is an epigenetic factor in brain maturation. Its role is to synchronize developmental events by modulating the expression of aiready transcribed genes in a time- and region-dependent fashion. The genes regulated by thyroid hormone do not belong to a single functional category. Some of them are likely to represent primary actions of the hormone exerted directly at the genomic level. Others may be secondary to a more general effect on cell differentiation. Most of the genes are only transiently affected by hypothy roidism and thyroid hormone treatment during a critical period Several questions remain unanswered, such as the role of specific thyroid hormone receptor isoforms and the molecular basis for the time window and the region-specific control of gene expression.