If you're happy with your former 401(k), you can leave your account where it is.
You can elect to roll over your traditional 401(k) into a traditional IRA.
“Many Americans have old 401(k) plans, which may not be properly positioned to help them prepare for retirement."
- From Take Charge of Your 401(k)
If you switched jobs or are preparing to, download your complimentary ebook to help you decide what to do with your 401(k).