In 52 BC, Pompey was made ‘consul without a colleague’ on the initiative of M. Porcius Cato and M. Calpurnius Bibulus. This moment is sometimes regarded as signalling an alliance between Pompey and the optimates. Others argue that it was no more than a necessary but temporary expedient. This chapter draws on detailed evidence surrounding the trial of T. Annius Milo to make the case for real co-operation between Pompey and ‘Cato’s circle’ in 52. Most obviously, Cato’s ally L. Domitius Ahenobarbus presided over the trial, while Cato himself served as juror. Moreover, though a supporter of Milo, Cato conducted himself so as not to undermine Pompey’s authority or the terms of the trial, which excluded any defence that Milo had acted for the good of the state. Cooperation did not end with Milo’s trial. Indeed, Cato helped to shape Pompey’s legislative programme as sole consul, even while Pompey continued to support Caesar. Tracking these and other alignments at work in 52 provides insights into the nature of political alliance in Republican Rome and the potential for constructive collaboration both through and despite pre-existing personal relationships.