Disorders of white matter are some of the most commonly encountered conditions in the practice of child neurology. For a child presenting with evidence of neurological impairment, a magnetic resonance imaging (MRI) of the brain is usually performed and often proves informative in suggesting the diagnosis. Traditionally, primary white matter disorders characterized by progressive destruction of myelin sheaths and to some extent oligodendrocytes, while initially preserving other components of the central nervous system (CNS), have been termed "leukodystrophies" when a genetic etiology is determined or suspected. The last decade has seen the identification of leukodystrophies with distinct clinical, MRI, and molecular findings. This review focuses on two disorders, Alexander disease (AD) and Megalencephalic leukoencephalopathy with cysts (MLC), which are newly characterized leukodystrophies that point to an important role for astrocytes in maintaining normal myelination. Alexander disease is an autosomal dominant disorder resulting from mutations in the GFAP gene that encodes for an intermediate filament protein specific to astrocytes. MLC is an autosomal recessive disorder in which 70% of cases harbor a mutation in MLC1, a gene that encodes for a protein highly expressed in distal astroglial processes. The precise pathogenesis of leukodystrophy in these two disorders remains unresolved. (Contains 2 tables and 2 figures.)