The Merger Guidelines released in March 2008 by the ACCC provide a guide to the analytical approach the ACCC intends to adopt to assessing mergers for the purposes of s 50 of the Trade Practices Act. The new guidelines do a relatively good job in listing the factors that the contemporary economic literature identifies as potentially characterising mergers that reduce competition and harm consumer welfare.However, unlike the earlier guidelines, they rarely explain the mechanism connecting the factors to the harm, and the conditions that need to be met for that harm to occur. This article provides a 'user's guide' to the guidelines that explains the reasoning that underpins the guidelines' assertions, and draws attention to the assumptions on which those assertions rest. We also provide an economic assessment of the guidelines and recommend a simpler criterion by which the ACCC should judge mergers.