Brain size variation among different mammals is tightly associated with different levels of cerebral function. Mechanisms that regulate the number of neurons and hence the size of the brain must be at least partially embedded within the very early phase of neocortical development, that is, embedded in proliferation/differentiation characteristics of the neural progenitor cells (NPCs) of the neocortex. Here we review a sequence of critical events through which the neocortex is formed in the embryonic forebrain, with particular emphasis on cell cycle kinetics of the NPCs that produce non-GABAergic projection neurons, the majority of neurons in the neocortex. In general, the critical parameters that determine the total number of cells produced by a given progenitor population through a sequence of cell cycles are (1) the number of cell cycles that constitute the production period and (2) the probability of cell cycle exit (Q fraction or Q) of progenitor cells for each of the cell cycles. We will also review molecular mechanisms that modulate the critical parameters above, with a special reference to the cell cycle regulatory protein p27(Kip1), inhibitor of G1 phase progression of the cell cycle. Finally the neocortical dysgenesis caused by genetic modification in mice where p27(Kip1) is either deleted or overexpressed is presented as examples of neuron number changes and resultant neocortical dysgenesis by Q fraction alteration.