What should I do with the 401(k) from a job I just left?

You generally have four options for an old 401(k): leave it in the former employer's plan, roll it into your new employer's 401(k), roll it into an IRA, or cash it out — and cashing out is almost always the worst choice because it triggers income tax plus a 10% penalty if you're under 59½. Rolling into an IRA gives you the widest investment menu and full control; rolling into your new 401(k) keeps things consolidated and preserves certain protections. Leaving it put can be fine if the plan has good low-cost funds. The biggest mistake is forgetting about it entirely — orphaned 401(k)s often sit in high-fee default options for years. At minimum, locate the account and confirm it's invested. This is a broad overview; the rollover mechanics and Roth-versus-traditional details deserve their own deeper look.