Why have central banks become more accountable and transparent in recent years? This paper considers a set of analytical models of monetary policy institutions to shed light on this. One conclusion it reaches is that uncertainty - regarding the central banks inflation preferences or about the underlying model of the world - can generate inflationary problems which transparency can help counteract. This offers one rationale for the current monetary policy framework in the UK. The paper also constructs a quantitative index of accountability. This suggests that transparency has been pursued most actively by central banks with little independence and a low accrued stock of credibility. Again, this chimes with UK experience. A shorter version of this paper is forthcoming in "Toward more effective Monetary Policy" - proceedings of the Seventh Internal Conference sponsored by the Bank of Japans Institute for Monetary and Economic Studies.