Should I pay off debt or invest first?

It depends on the interest rate. A useful rule of thumb: if the debt interest rate is above 7%, prioritize paying it off — that return is guaranteed and risk-free. If the rate is below 4–5% (common with mortgages or subsidized student loans), you can often do better by investing in index funds over a long horizon. For rates in between (5–7%), consider splitting: contribute at least enough to get your full 401(k) employer match (that's an instant 50–100% return), then tackle the debt. High-interest credit card debt (15–25%) should almost always be paid off before investing beyond the employer match.