Most mitochondrial proteins are synthesized as precursor proteins in the cytosol and post-translationally transported into mitochondria. The mitochondrial surface protein Tom70 acts at the interface of the cytosol and mitochondria. In vitro import experiments identified Tom70 as targeting receptor, particularly for hydrophobic carriers. Using in vivo methods, we revisited the Tom70 function, confirming that it supports the biogenesis of carriers and other mitochondrial proteins which considerably expands the set of Tom70-dependent mitochondrial proteins. However, the crucial activity of Tom70 is its recruitment of cytosolic chaperones to the outer membrane. Surprisingly, tethering an unrelated chaperone-binding domain onto the mitochondrial surface complements most of the defects caused by the absence of Tom70. In vivo, small single spanning inner membrane proteins can be particularly problematic when present in excess and the chaperone-binding ability of Tom70 is crucial to suppress their proteotoxicity. Thus, the predominant function of Tom70 is more that of a cytosolic co-chaperone, and its role as mitochondria-specifying receptor apparently is of less immediate relevance.