Should I add small-cap and mid-cap funds to my portfolio?

It's optional — and if you already own a total-market fund, you have them. Small-cap stocks are smaller companies and mid-caps sit between small and large; both have historically delivered slightly higher long-run returns than large caps, paired with bigger swings along the way. A total U.S. market index fund already holds small, mid, and large caps in their natural market proportions, so you're covered without buying anything extra. Some investors deliberately overweight small caps (a "small-cap tilt") hoping to capture that historical premium, but the premium is unreliable and can vanish for a decade. If you hold an S&P 500 fund instead of a total-market fund, adding a small/mid-cap fund fills the gap those 500 large companies leave out. Keep any deliberate tilt modest. Build the mix at wealthserene.com/tools/portfolio-builder.