The forerunner of the human cerebral hemispheric white matter is the intermediate zone, situated between the ventricular zone and cortical plate. It is traversed radially by migrating cell precursors of neurons, glia, and microglia originating in the ventricular zone and germinal matrix, according to a complex timetable beginning in the first trimester (see Chapters 19–21). Full completion of white matter development, that is, full myelination, continues into the third decade of postnatal life, representing one of the longest maturation tempos of any structure in the human body. The intermediate zone also hosts the axonal processes of projection neurons, which elongate and later find their way to their sites of synapse, relying on help from subplate neurons, also locally present in the region. Oligodendroglial progenitors populating the intermediate zone undergo in parallel their own metamorphoses, to eventually synthesize myelin proteins and lipids, and progressively enwrap the axons to optimize their conductance. In this chapter, we will review briefly the elements of this process that are important for understanding later pathologies involving the fetal and infant white matter (Chapter 34).