This chapter covers the leading theories of the markets. The dominant theory of the Efficient Market Hypothesis distracted regulators, market participants and central bankers from paying attention to market prices as signals or from recognizing the existence of bubbles in the housing market, as Alan Greenspan admitted. The behavioural theorists shift the emphasis away from examining trends in the market data and developing models to explain them, to the behaviour of investors in the market, or rather to the factors influencing their behavior. There are two building blocks of behavioural finance: one is that in an economy where rational and irrational traders interact, 'irrationality can have a substantial and long-term impact on prices'. The second building block is psychology. Swedburgh's paper is important in that it points out that trust underlies the smooth, or one might say, efficient functioning of the market.