Asset allocation is the process of dividing your investment portfolio among different asset classes such as stocks, bonds, and cash equivalents. It is widely considered the most important decision an investor makes, with studies showing that asset allocation accounts for the vast majority of portfolio return variability over time.

The classic age-based rule of thumb suggests holding a stock percentage equal to 100 minus your age, with the remainder in bonds and cash. A 30-year-old would hold 70% stocks and 30% bonds. This heuristic works because younger investors have more time to recover from downturns, while older investors need more stability as they approach or enter retirement.

Risk tolerance is equally important. A conservative investor may sleep better at night with fewer stocks even if they are young, while an aggressive investor with a long timeline and high risk tolerance may tilt heavily toward equities. The key is finding a mix you can stick with through both bull and bear markets without making emotional decisions that harm your long-term returns.