Our main result is that fluctuations in market belief are large contributors to the time variability of risk premia. We study the premia on long positions in Federal Funds Futures, 3-month and 6-month Treasury Bills. On average, the risk premium on holding such assets for 6-12 months is about 40-60 basis points. We show that over 50% of the time market beliefs contribute more than 25-30 basis points to the premium. Moreover, the time variability of market belief is so large that this contribution is frequently larger than 50 basis points. As to the structure of the premium we show that when the market holds abnormally favorable belief about an asset’s future payoffs the market views the long position as less risky and hence the risk premium on that asset declines. Generalizing to the market as a whole we show that periods of market optimism (i.e. bull markets) are periods when the market risk assessment falls while in periods of pessimism the market’s risk assessment rises. That is, fluctuations in risk premia are inversely related to the degree of market optimism about future prospects of asset payoffs.