It is well established that in most species, the hippocampus shows extensive postnatal development. This delayed maturation has a number of implications, which can be thought of in three categories. First, the late maturation has the direct effect of depriving the developing organism of at least some of the functions of the hippocampus, in particular place learning, context coding and in humans, episodic memory. Second, such learning that does occur very early in life, prior to hippocampal maturation, will largely bear the imprint and properties of those brain systems that, unlike the hippocampus, are fully functional early in life. Third, the active state of development of hippocampus in the first weeks and months of life render this structure susceptible to disruption by environmental and/or chromosomal factors. In this article, I discuss my efforts, with many colleagues over the past 40 years, to understand each of these implications.