The most common reason for changing your asset allocation is a change in your time horizon. In other words, as you get closer to your investment goal, you'll likely need to change your asset allocation. For example, most people investing for retirement hold less stock and more bonds and cash equivalents as they get closer to retirement age. You may also need to change your asset allocation if there is a change in your risk tolerance, financial situation, or the financial goal itself.

But savvy investors typically do not change their asset allocation based on the relative performance of asset categories - for example, increasing the proportion of stocks in one's portfolio when the stock market is hot. Instead, that's when they "rebalance" their portfolios.

Rebalancing 101

Rebalancing is bringing your portfolio back to your original asset allocation mix. This is necessary because over time some of your investments may become out of alignment with your investment goals. You'll find that some of your investments will grow faster than others. By rebalancing, you'll ensure that your portfolio does not overemphasize one or more asset categories, and you'll return your portfolio to a comfortable level of risk.