After you’ve switched employers, it’s time to think about what to do with the old retirement account(s) (401k/403b/457) you previously contributed to. But before you do anything, make sure any company match dollars you’ve earned have vested appropriately. Vesting schedules (i.e. when the money is yours free and clear) vary, and vesting may occur immediately, with some fixed percentage per year employed, completion of a specified residency program or contract period, etc.
As a personal anecdote, my transitional year internship had a 50% match up to 4% of salary with 100% vesting after the completion of a residency program (our TY qualified, as it was considered a complete 1-year program). I noticed that the vested portion of my employee contribution was still zero dollars after finishing two years ago, so I eventually got around to emailing them, they looked into it, realized the error, and contacted the plan: now I have my money. Now, my former employer was very responsive, and I doubt very much that this was done on purpose, but there is no denying that it would be in a company’s best interest to conveniently forget to vest their matching contributions and see who notices. Just saying.
Anyway, once you have all your hard earned money under your name, you have three real choices: cash out the plan, keep it where it is, or transfer/rollover to a new account.
Option 1: Spend it
No. Don’t cash out the plan. You’ll pay both federal and state taxes on it like it’s regular income PLUS an additional 10% early withdrawal penalty (because you’re probably not 59.5 years old). Your tax-sheltered retirement account distributions are limited on an annual basis (i.e. you can only contribute 18k in 2015/16), so why waste the benefits and tax-free growth?
Option 2: Leave it
Leave the money where it is. Nothing wrong with this, but your former employer’s plan may not have the best fund options or the lowest fees. Some employers also won’t let you do this if your balance is low, and others may hike up your fees without giving you a solid heads up once you’re no longer part of the team. Bottom line: if your old plan doesn’t have low management fees with access to low-cost index funds, then it’s not a great place for your money.
Option 3: Move it
For many people, moving it elsewhere is the best plan.
Your new employer
You can usually transfer funds into your new employer’s fund (assuming they accept rollovers), which is a good idea if your employer’s plan is better. But if the fees or fund options aren’t better, then the main advantage to a transfer in this setting is having fewer accounts to keep track of.
Roth IRA
If your income is within the Roth limits (which it almost certainly is as a resident or fellow), you can roll over a regular 401k/403b to a Roth IRA (you’ll pay taxes on the conversion, but then it’ll be tax-free on withdrawal). This may be the best option as a resident (if you have the money on hand to pay the taxes on it) and a good in general, particularly if your current income is lower than you expect during retirement. You can also always convert a Roth 401k/403b to Roth IRA with no penalty (Roth to Roth conversions are always Kosher), so if you have a Roth 401k/403b, just do this.
Individual (Solo) 401k
If you moonlight or have any self-employment 1099 income, then you could transfer your old money into a solo 401k that you set up for your side business’ income.1 Individual 401ks are pretty awesome. While you can still only contribute up 18k per year as an individual, your business can also offer up 20% of its (your) profits up to the 51k limit (which is a per business limit, not a per person limit).2 Vanguard, one of the best solo 401k options, doesn’t allow for 401k rollovers, but low-cost competitor Fidelity does. An individual 401k of your choosing should have lower fees and good fund options compared with most employer plans.
Traditional IRA
Lastly, you could roll it over into a traditional IRA. But putting pre-tax money into a tIRA means that if you attempt the “backdoor Roth” in the future (which you should/will), you’ll eventually have pay tax on the conversion. So probably not the best unless you then roll over the IRA into an employer’s 401k (that takes rollovers) in the future. There’s no reason to do this really unless both your old and current employer’s options have high fees and you don’t have any 1099 income at the moment to set up your own 401k.
The internet has a gazillion pages dedicated to this question. Here is some good further discussion of the options and their relative merits.