This paper probes the limits of transparency in monetary policymaking along two dimensions: feasibility and desirability. It argues that, due to limited knowledge about the economy, even central banks (CBs) that are considered champions of openness are not very clear about their measurement of the output gap and about their beliefs regarding the effect of policy on inflationary expectations. Consequently, feasibility constraints on transparency are more serious than stylized models of the transmission mechanism would imply. In addition, no CB has made clear statements about its objective function, including in particular the relative weight on output versus inflation stabilization, the policy discount factor and the shape of losses from the inflation and the output gaps over the possible ranges of realizations of those variables.The paper also argues that there is a trade-off between full transparency and full utilization of information in setting policy and that excessive transparency may facilitate the exertion of political pressures on the CB.The last section of the paper abstracts from feasibility constraints and discusses the desirable levels of openness in various areas of the policymaking process. It is argued that the strongest case against immediate transparency arises when the CB has private information about problems within segments of the financial system. Premature release of information may, in such a case, destroy efficient risk-sharing arrangements and long-term investments by triggering a run on the financial system. This is illustrated within the context of the classic Diamond-Dybvig model of bank runs. The paper also probes the desirable levels of transparency in other areas of the policymaking process like the bank's objective function, the bank's output target, forecasts of economic shocks, disagreements within the CB board and the publication of CB interest-rate forecasts.