This thesis explores the implications of imperfect information for monetary policymaking. It considers the central bank as the coordinator of expectations because of its competence rather than its commitment to a low and stable inflation. By competence, we intend to say the ability to correctly forecast the future state of the economy. The central point has been to show that competence together with communication enables influence, and that central bank influence of private expectations enables to loose monetary policy constraints to reach its macroeconomic objectives. Chapter 1 proceeds to an empirical review of the vast literature dealing with the relative forecasting performance of the Federal Reserve, a central bank which publishes its forecasts with a 5-year lag. Chapter 2 focuses on central banks which publish their forecasts in real-time what allows for emphasizing the expectations channel of monetary policy and the question of credibility through the link between relative forecasting performance and influence of central banks. We propose to define endogenous credibility as the capacity to influence arising from a superior forecasting performance, in opposition to exogenous credibility for which central banks need not a forecasting advantage to be influential. Chapter 3 investigates the theoretical implications of endogenous influence for monetary policymaking through a New-Keynesian economy with non rational expectations. Last, Chapter 4 assesses the monetary policy preferences of three central banks which have adopted the inflation targeting framework and therefore communicate their forecasts in real-time.