The peroxisomal membrane forms a permeability barrier for a wide variety of metabolites required for and formed during fatty acid beta-oxidation. To communicate with the cytoplasm and mitochondria, peroxisomes need dedicated proteins to transport such hydrophilic molecules across their membranes. Genetic and biochemical studies in the yeast Saccharomyces cerevisiae have identified enzymes for redox shuttles as well as the first peroxisomal membrane transporter. This peroxisomal ATP-binding cassette transporter (Pat) is highly homologous to the gene mutated in X-linked adrenoleukodystrophy (X-ALD). The yeast Pat is required for import of activated fatty acids into peroxisomes suggesting that this is the primary defect in X-ALD.