Unlike some other retirement plans, with a Savings Incentive Match Plan for Employees, or SIMPLE IRA, you can take it all with you, even if you've only worked for your employer for a short time.
Most employer retirement plans come with a vesting schedule, meaning that while your contributions are always yours, your employer's contributions are only yours after a certain period of time. One nice benefit of a SIMPLE IRA is that those employer contributions are 100 percent vested immediately.
But what can you do with it? As in most things financial, it depends. One hitch to the "you can take it with you" rule is that if it hasn't been at least two years since you first participated in the plan, you'll face a hefty 25 percent premature withdrawal penalty (if you're under age 59.5 and no exception applies), plus regular income tax for moving it into anything other than another SIMPLE IRA. Ouch.
So it pays to leave it where it is until your two years is up. After that, you are free to rollover your SIMPLE into your own IRA and invest it as you wish. It may be possible too to roll it into your new employer's 401(k) plan. That will depend on the new plan's rules.
When rolling over your SIMPLE, you'll want to be sure the money goes directly from the SIMPLE to its new destination, without passing through your hands. It's okay to have the check sent to you, but it should be made payable to the new custodian and not you to avoid any tax complications.
What you don't want to do is cash it in, tempting as it may be. Assuming you're under 59.5, without one of the exceptions like qualifying medical expenses or college tuition, you'll be hit with the double whammy of a premature distribution penalty (either that ugly 25 percent or the usual 10 percent) plus income tax, not to mention you're taking the funds out of play for retirement savings growth.
Unless you absolutely need the money, it's best to keep it tucked away for retirement.