It extended Modern Portfolio Theory to introduce the notions of systematic and specific risk.
All investors have identical investment horizons.
All investors have identical perceptions regarding the expected returns, volatilities and correlations of available risky investments. All investors will hold the market portfolio, leveraging or deleveraging it with positions in the risk-free asset . CAPM divides the risk of holding risky assets into systematic and specific risk. Systematic risk is the risk of holding the market portfolio. As the market moves, each individual asset is more or less affected.