The Bogleheads advise that after you determine your asset allocation (stock %/bonds %), then you apply the percentages to your overall portfolio, wherever the funds are. So, if you are overweight on bonds, say by 10%, if you are contributing to your accounts regularly, then increase the amount you have set for stock. If the imbalance can be solved fairly quickly by this method, then you are at your AA. If not, you would need to sell some bond funds and buy the stocks. Reallocation can be emotionally difficult to do if, say, if bonds are doing great and stocks are doing poorly. You would be selling a winner, but that is how it works to rebalance. Some people rebalance yearly or semi-yearly; some do it when the AA is off by over 5% or so.