Aces can be either high or low! A run consisting of an Ace, a Two, and a Three is valid (as long as they share a suit); a run consisting of a Queen, a King, and an Ace is also valid. 

However, Aces cannot be both high and low at the same time: a run consisting of a King, an Ace, and a Two is not valid.

When cards are scored, an Ace is worth 11 points.