We present a definition of factor structure that is less restrictive than the one typically used in arbitrage pricing models. Our factor structure restrictions build on the following intuitive distinctions between factor variance and idiosyncratic variance: (i) A well-diversified portfolio contains only factor variance. (ii) If a portfolio is uncorrelated with the well-diversified portfolios, then it contains only idiosyncratic variance; so if a sequence of such portfolios becomes well-diversified, the limiting variance should be zero. Our factor structure restrictions imply Ross' [5] arbitrage pricing formula. We obtain upper and lower bounds on the approximation error in that formula; these bounds may be useful in empirical work. They imply that arbitrage pricing is exact if and only if there is a risky, well-diversified portfolio on the mean-variance frontier. If all mean-variance efficient portfolios are well-diversified, then the well-diversified portfolios provide mutual fund separation. Our factor structure restrictions are satisfied (with K factors) if and only if the covariance matrix of asset returns has only K unbounded eigenvalues as the number of assets increases.