The other day while playing, I asked one of my opponents how many cards he had in his hand, he asked me if it where in the rules that he had to tell me, I couldn't find it anywhere in the rulebook, and we began discussing whether or not you had to tell this to your opponent, whether or not you only had to tell your opponent when he had a power that gave him the option of taking cards, hands etc. from his opponents as this is quite an important factor when you have a power of that type, if you only had to tell when a mainplayer asked you and so forth.

But we could'nt find it covered anywhere in the rulebook - have I missed something, or how do you handle this?

Furthermore I have a question about when you should zap a power - if it is an optional power, do you have to zap it BEFORE it is announced that it will be used, or can you zap it at the moment it is announced it will be used?

Number of cards held is always public knowledge. I don't recall if there is any mention of this explicitly in the rules, but this is how the game has always worked (see the online game for confirmation) and a number of aliens rely on this knowledge. For example, the Mutant can't use his power to take two cards from me if he doesn't know for certain that I have at least two cards, can he?

As for zapping optional powers, you can only zap them when they're announced. You cannot preemptively zap an optional power. Zaps are played in response to the use of power.

Old post, but new member. So just in case anyone new is reading this and wondering:

I'm fairly sure its safe to say the number of cards in a person's hand is open to all Players. I say this because I seem to recall a power or maybe a flare somewhere that made a point of stating their hand is hidden from sight. This would imply that everyone elses is not.

Or so my feeble memory says.

I'd have to disagree, Adam. I'd say that if a power to retrieve cards from an opponents hand is necessary THEN it would become an issue, however I don't believe that knowing an opponents ability to engage in an encounter (by the number of cards he has) is public knowledge. If a player runs out of cards, he cannot draw a new hand unless he becomes a main player. This would imply that the need (or knowledge) of cards one possesses is not central to the turn of the game, to that player or even the need for other players to have knowledge of.