1. A system for assisting a rescuer in performing resuscitation activities on a patient, comprising: at least one chest compression sensor configured to sense movement of the patient's chest; a second sensor configured to sense information indicative of whether the rescuer's hand is in contact with or is not in contact with the patient's chest, wherein the second sensor is configured to be positioned on the patient's chest at a location corresponding to a location of the rescuer's hand when delivering chest compressions to the patient's chest; and a processor configured to: receive and process information from the chest compression sensor and the second sensor; identify occurrence of a chest compression based on information from the second sensor; and calculate chest compression depth based on the information sensed by the chest compression sensor during the identified chest compression.