Credit cycles have been a characteristic of advanced economies for over 100 years. On average, a sustained pick-up in the ratio of credit to GDP has been highly correlated with banking crises. The boom phases of the cycle are characterised by large deviations in credit from trend. A range of mechanisms can generate these effects, each of which has strategic complementarity between banks at its core. Macro-prudential policy could curb these credit cycles, both through raising the cost of maintaining risky portfolios and through an expectations channel that operates via banks' perceptions of other banks' actions.