What is the minimum re-raise in Texas hold em? I have heard both that you need to double the previous bet and that the minimum raise is the big blind. Which is it?
In limit games, the only raise amount allowed is the big blind during preflop and flop play; double the big blind during turn and river play.
The correct minimum raise in big bet games (no limit and pot limit) is to increase the amount of the bet by the amount of the previous bet (e.g. double the previous raise*), or to raise all-in if you do not have enough in your stack to actually double the previous raise. Reference: http://www.learn-texas-holdem.com/how-large-is-the-minimum-raise.htm
Note that doubling the previous raise is different than doubling the previous bet. If I were to bet 100 and you raised to 300, someone could raise to 500, even though it would take 600 to double your bet.
In the case of an all-in, there is a distinction between what it allowed for the raising player, and what constitutes a raise to the rest of the players at the table. If a bet is considered to be a full raise, it reopens the betting, allowing another player to raise again if they choose. In the case of an all-in for less than the minimum raise, the all-in is allowed, but it does not constitute a full raise, and as such it does not reopen betting. In many cases, betting will reopen for an all-in raise that is a fraction of a full raise as well, though this varies a bit from casino to casino. Typically, if this is the case, the most common amount required to reopen betting is 1/2 of a full raise.
As noted in the comments on the question, one place where the definition of what constitutes a minimum raise is in the situation where a three-way pot occurs with the smallest stack acting third and shoving all-in for less than a true raise. That is: Player A raises to 300, then B raises to 500, then C shoves for 599. If A calls, then in most casinos the only options B has left are to call or fold; since no one made a full raise after his last raise, he is not permitted to raise again. Had C shoved for 600 or more, many casinos (but not all) casinos would allow B to raise again.