You can also make after tax contributions to a 401K, at least the ones I've dealt with. It's a different limit than the before tax contributions. They can all go into the same 401K fund at your employer but are tracked separately for tax purposes once you get to retirement age and start taking money out, much the same as many company plans track your contributions separately from company match funds. In the case of the fellow with $20 million, I would expect that some of that was after-tax contributions, and much of that was also good stock performance, price as well as splits.