In this paper, we introduce a novel medium access control (MAC) protocol for Radio Frequency Identification (RFID) systems which exploits the statistical information collected at the reader. The protocol, termed Adaptive Slotted ALOHA Protocol (ASAP), is motivated by the need to significantly improve the total read time performance of the currently suggested MAC protocols for RFID systems. In order to accomplish this task, ASAP estimates the dynamic tag population and adapts the frame size in the subsequent round. We demonstrate that ASAP provides significant improvement in total read time performance over the current RFID MAC protocols. We extend the design to mobile RFID systems where tags move at constant velocity in the reader's field, and show that ASAP performs well in mobile scenarios as well.