In an increasingly globalised and digitised world economy, the number of mergers transactions that impact more than one country has also increased. For competition authorities responsible for reviewing merger transactions, this has created new challenges and introduced more complexity to their merger review procedures and analyses. This paper surveys these challenges, explains the reasons why competition authorities may arrive at different decisions, and discusses the role that international co-operation plays in each phase of a cross-border merger review. Drawing from a range of case studies across both OECD and non-OECD member countries, the paper highlights practical tools competition authorities can use to improve the effectiveness of their cross-border mergers.