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Why would you not want to rollover a previous employer's 401(k) when changing jobs?
The biggest reason why one might want to leave 401k money invested in an ex-employer's plan is that the plan offers some superior investment opportunities that are not available elsewhere, e.g. some mutual funds that are not open to individual investors such as S&P index funds for institutional investors (these have expense ratios even smaller than the already low expense ratios of good S&P index funds) or "hot" funds that are (usually temporarily) closed to new investors, etc. The biggest reason to roll over 401k money from an ex-employer's plan to the 401k plan of a new employer is essentially the same: the new employer's plan offers superior investment opportunities that are not available elsewhere. Of course, the new employer's  401k plan must accept such roll overs. I do not believe that it is a requirement that a 401k plan must accept rollovers, but rather an option that a plan can be set up to allow  for or not.  Another reason to roll over 401k money from one plan to another  (rather than into an IRA) is to keep it safe from creditors. If you are  sued and found liable for damages in a court proceeding, the plaintiff can come after IRA assets but not after 401k money.  Also, you can take a loan from the 401k money (subject to various rules about how much can be borrowed, payment requirements etc) which you cannot from an IRA. That being said, the benefits of keeping 401k money as 401k money must be weighed against the usually higher administrative costs and usually poorer and more limited choices of investment opportunities  available in most 401k plans as Muro has said already.