Mitochondria consist of two membranes and around thousand different proteins. Most mitochondrial proteins are encoded by the nuclear DNA, synthesized in the cytosol, and imported into and sorted within mitochondria with the aid of translocators in the mitochondrial membranes. Translocators function as a receptor for recognition of destination signals, provide a protein conducting channel, and drive vectorial translocation and unfolding of substrate mitochondrial proteins. This review provides an overview on the current understanding of the structure and functions of the TOM40 complex and TIM23 complex, two of the four major translocators in mitochondria.