Gene expression is one of the main molecular processes regulating the differentiation, development, and functioning of cells and tissues. In this review a handful of relevant terms and concepts are introduced and the most common techniques used in studies of gene expression/expression profiling (also referred to as studies of the transcriptome or transcriptomics) are described. The main foci of this review are the advancements in studies of the transcriptome in the human brain, the transcriptome's variability across different brain structures, and the systematic changes that occur through different developmental stages across the life span in general and childhood in particular. Finally, the question of how the accumulating data on the spatial and temporal dynamics of the transcriptome may shed light on the molecular mechanisms of the typical and atypical development of the central nervous system is addressed.