Any portfolio allocation is subject to drift: As some investments outperform others, the makeup shifts over time to include greater weightings in outperforming assets. But drift can significantly increase a portfolios exposure to certain asset classes or investments, in effect increasing its risk and misaligning with target allocations. Rebalancing can be accomplished by either selling investments in the asset class that exceeds the target and buying investments in the underweighted asset or using new money to increase the underweighted asset. If you have multiple investment accounts, rebalancing will involve figuring how your money is divided among asset classes in each account and then across all accounts, whether in taxable brokerage, mutual fund, or tax-deferred accounts. Its a good idea to rebalance once a year or whenever your goals change. Alternatively, you may prefer to set a percentage limit of variance, say 5% on either side of your intended target, that would trigger a review and possible rebalancing.