C-2-Symmetric bis(oxazolinyl)pyridine (pybox)-Cu(II) complexes have been shown to catalyze enantioselective Mukaiyama aldol reactions between (blenzyloxy)acetaldehyde and a variety of silylketene acetals. The aldol products are generated in high yields and in 92-99% enantiomeric excess using as little as 0.5 mol % of chiral catalyst [Cu((S,S)-Ph-pybox)](SbF6)(2). With substituted silylketene acetals, syn reaction diastereoselection ranging from 95:5 to 97:3 and enantioselectivities greater than or equal to 95% are observed. Investigation into the reaction mechanism utilizing doubly labeled silylketene acetals indicates that the silyl-transfer step is intermolecular. Further mechanistic studies revealed a significant positive nonlinear effect, proposed to arise from the selective formation of the [Cu((S,S)-Ph-pybox)((R,R)-Ph-pybox)] (SbF6)(2) 2:1 ligand:metal complex. A stereochemical model is presented in which chelation of (benzyloxy)acetaldehyde to the metal center to form a square pyramidal copper intermediate accounts for the observed sense of induction. Support for this proposal has been obtained from double stereodifferentiating reactions,EPR spectroscopy, ESI spectrometry, and, ultimately, the X-ray crystal structure of the aldehyde bound to the catalyst. The Ct-symmetric bis(oxazolinyl)-Cu(II) complex [Cu((S,S)-tert-Bu-box)](OTf)(2) is also an efficient catalyst for the aldol reaction, but the scope with this system is not as broad.