Little has been written on deputy-leaders and the received wisdom, such as it is, is that deputy-leaders have little power and hence do not matter. A global survey of deputyleaders found that 68 per cent of states had a deputy-leader. So, however powerful they may be, they are certainly a fairly common political phenomenon. To test whether or not deputy-leaders are politically powerful and thus matter, seven hypotheses were identified with nine observable implications. A comparative approach was adopted, examining the careers of 64 deputy-leaders in five states. The overall results of the tests were somewhat at odds with the perceived wisdom that deputy-leaders do not matter. Furthermore, the outcomes of the tests at the level of the individual states in this study found strong proof that deputy-leaders in the US can influence policy outcomes and there was some proof that British and Swedish deputy-leaders could do so as well.