Air (R-729) is free and plentiful and there are no direct global warming, ozone depletion, and toxic potentials associated with it. In view of these attributes, air cycle air conditioning (referred to as the R-729 system) commends itself prima facie against the backdrop of today's environmental concerns. Presented in this paper are the historical developments and a critical assessment of the R-729 system for motor vehicles including its coefficient of performance (COP) under realistic operating conditions and the total equivalent warming impact (TEWI). Also included in the paper are the extensive performance comparisons of the R729 system with those of the highly-touted carbon dioxide (R-744) system and the baseline R-134a system. As elicited by the following tabular values of COP and TEWI, both the R-744 and R-729 systems are inferior to the baseline R-134a system.