y v.vassiliadis@fme.aegean.gr z g.dounias@aegean.gr This paper considers the task of forming a portfolio of assets that outperforms a benchmark index, while imposing a constraint on the tracking error volatility. We examine three alternative formulations of active portfolio management. The¯rst one is a typical setup in which the fund manager myopically maximizes excess return. The second formulation is an attempt to set a limit on the total risk exposure of the portfolio by adding a constraint that forces a priori the risk of the portfolio to be equal to the benchmark's. In this paper, we also propose a third formulation that directly maximizes the e±ciency of active portfolios, while setting a limit on the maximum tracking error variance. In determining optimal active portfolios, we incorporate additional constraints on the optimization problem, such as a limit on the maximum number of assets included in the portfolio (i.e. the cardinality of the portfolio) as well as upper and lower bounds on asset weights. From a computational point of view, the incorporation of these complex, though realistic, constraints becomes a challenge for traditional numerical optimization methods, especially when one has to assemble a portfolio from a big universe of assets. To deal * We are thankful to the two anonymous referees for their valuable comments and suggestions.New Mathematics and Natural Computation Vol. 5, No. 3 (2009) 535À555 # . c World Scienti¯c Publishing Company 535 New Math. and Nat. Computation 2009.05:535-555. Downloaded from www.worldscientific.com by UNIVERSITY OF QUEENSLAND on 08/15/15. For personal use only.properly with the complexity and the \roughness" of the solution space, we use particle swarm optimization, a population-based evolutionary technique. As an empirical application of the methodology, we select portfolios of di®erent cardinality that actively reproduce the performance of the FTSE/ATHEX 20 Index of the Athens Stock Exchange. Our empirical study reveals important results concerning the e±ciency of common practices in active portfolio management and the incorporation of cardinality constraints.