This chapter studies the characteristics of the most important and well-known factors. Factor portfolios are portfolios of stocks based on certain characteristics, such as the size of the company, the price of the stock in relation to, e.g., the earnings of the company, the sector within which the firm operates, etc.Factors that perform better than the overall stock market tend to suffer more during recessions. To compensate investors for their underperformance during recessions, returns on these factors during expansions are so high that average stock returns over the full business cycle end out being high. Conversely, those factors that provide lower average returns than the overall stock market do so because they perform relatively better during recessions. The business cycle again plays an important role for understanding stock-market patterns.