In most jurisdictions, the parties to a merger are not allowed to implement the transaction prior to receiving clearance from the relevant competition authority. Competition authorities have become increasingly vigilant about enforcing this standstill obligation. Recent high profile cases involving record fines provide some insight into which activities may be regarded as impermissible 'gun jumping' and which measures are permitted in preparing to implement a merger. This article analyses the European Commission's decision in Altice and the Court of Justice's judgment in Ernst & Young and draws conclusions, in particular, on the implications for information exchange between merging parties and the identification of 'ordinary business', which must remain unaffected until closing of a prospective concentration.