In this paper, we first outline the foundations in economic theory of so-called coordinated effects with a particular view to mergers and with a special focus on transparency. Then, we review a number of seminal merger cases in EU competition policy (Airtours, Sony/BMG, ABF/GBI Business) in light of that theory. Next, we discuss in more detail a recent Danish merger prohibition that was based on the presence of coordinated effects. This case poses special challenges to the theory of coordinated effects, because a very large number of products were involved, and significant, individualized discounts were widespread in the market in question. Finally, we briefly present a few other cases in which transparency has been an issue.