The ECB has adopted a variety of different unconventional monetary policy measures since the Global Financial Crisis. In this Briefing Paper, we assess the effectiveness of unconventional monetary policy measures based on a review of the empirical literature and on theoretical considerations. The effectiveness of these measures depends on the specific circumstances when they are implemented. Therefore, we also discuss whether the effectiveness of unconventional monetary policy may change over time, taking into account that the available empirical literature on this issue is still limited at this time. In this regard, we also discuss potential risks and side effects associated with unconventional monetary policy, focusing on those side effects that currently may directly counteract the expected impact of monetary policy on output and inflation in the euro area.