There is no right or wrong answer to that question. Here are the pros and cons of each of the options you stated above:1. Leave it where it is. While this is clearly the simplest option, it is also the least favorable. For one, you cannot make any more contributions to your 401(k) if you leave it where it is. And, more likely than not, you will not be kept up to date with changes in the investment options or performance of the plan, which can hurt you long-term performance. 2. Roll Into new companies 401(k)This is a good option for some people. First, you need to make sure that your new plan allows you to roll your old 401(k) into the new plan. Then, you need to ensure that the investment options you have available in your new 401(k) are at least as good as those in your old plan (if they are not, this may not be the best option)If you are able to, and the investment options are equal or better, there is a clear advantage to do this over leaving it at your old company. You will be able to update and monitor your investment much more easily, and you will have only 1 401(k) instead of 2. However, the downside to a 401(k), as others have mentioned, is that you have limited investment flexibility, 401(k) plans are designed to be simple. They are designed this way to encourage employees to contribute. Part of the way they are kept simple is by limiting investment options, so that you are not picking from 10,000 mutual funds or ETF's, but are picking from 10. The 10 funds provided in your 401(k) are likely diversified, but may be over-priced and/or under-performing relative to funds available outside your 401(k).3. Roll into a Roth IRARolling your old 401(k) has the advantage of increasing your investment flexibility. Within a Roth IRA, you have access to an almost unlimited array of investment options. However, because it is a roth IRA, you will be taxed on the entire 401(k) balance that was rolled over. The advantage, however, is that all withdrawals from your Roth IRA (taken after age 59 1/2) are tax free, unlike a traditional IRA where they are taxed. You are also able to make contributions to your Roth IRA if your modified AGI (adjusted-gross-income, don't ask me what this is unless you want another 10 page post) is within certain limits.4. Roll Into a Traditional IRARolling your old 401(k) into a traditional IRA has similar advantages to rolling it over toa Roth IRA, with the added benefit that you will not be taxed on any portion of the rollover (assuming that the entire rollover amount is deposited into your new IRA within 60 days of you taking with withdrawal). However, all of your withdrawals from the IRA will be fully taxable when you take them out, and may also incur a 10% penalty if taken out more age 59 1/2. You can make contributions to your IRA up to $5,000 ($6,000 if over age 50), and depending upon your income and whether or not you have a retirement plan at work, may be able to deduct the contributions on your tax return.5 Something elseYou probably shouldn't do something else. The only other big something else I can think of is withdrawing the money altogether. If you di this, you would be taxed no the entire 401(k) balance. and very likely have to pay an additional 10% penalty on the account balance.So which option is best? That depends.If your old employer has better investment options then you new plan, AND you are not comfortable choosing your own investments AND you do not want to hire a professional advisor, it may be best to leave your assets where they are. If you do this, make sure you take the time to keep up to date with what's happening with he plan, it's options and your investments.If your new employer has good investment options and allows you to roll over your old 401(k), AND you like the simplicity of the 401(k) AND want to manage it yourself, understanding the investments may not be the best, roll it over to your new employer 401(k)If you are comfortable choosing your own investments, or want to hire a professional advisor, roll it over into an IRA or Roth IRA (this process is very easy...if you have questions about that I would be happy to answer)I would highly recommend speaking with a CPA or financial advisor when choosing between Roth IRA and traditional IRA. Both have their purpose, but there is not one clear answer as everyone's financial situation is different.Sorry for the extremely long answer, but this is a topic people seem to be very confused about and there is no short--way to answer this question.I hope this helps!