The Constitution specifies that a majority of members constitutes a quorum to do business in each house. Representatives and senators rarely force the presence of a quorum by demanding quorum calls; thus, in most cases, debates continue even if a majority is not present.
The Senate uses roll-call votes; a clerk calls out the names of all the senators, each senator stating "aye" or "no" when his or her name is announced. The House reserves roll-call votes for the most formal matters, as a roll-call of all 435 representatives takes quite some time; normally, members vote by electronic device. In the case of a tie, the motion in question fails. In the Senate, the Vice President may (if present) cast the tiebreaking vote.