Primary seed dormancy is an important adaptive plant trait whereby seed germination is blocked under conditions that would otherwise be favorable for germination. This trait is found in newly produced mature seeds of many species, but not all. Once produced, dry seeds undergo an aging time period, called dry after-ripening, during which they lose primary dormancy and gradually acquire the capacity to germinate when exposed to favorable germination conditions. Primary seed dormancy has been extensively studied not only for its scientific interest but also for its ecological, phenological, and agricultural importance. Nevertheless, the mechanisms underlying primary seed dormancy and its regulation during after-ripening remain poorly understood. Here we review the principal developmental stages where primary dormancy is established and regulated prior to and during seed after-ripening, where it is progressively lost. We attempt to identify and summarize what is known about the molecular and genetic mechanisms intervening over time in each of these stages.