1. A method, implemented on a wearable fitness monitor comprising one or more motion sensors, one or more heartbeat waveform sensors, and one or more processors, the method comprising: (a) obtaining, by the one or more processors, a motion signature obtained using data from the one or more motion sensors of a wearable fitness monitor configured to be worn by a person, wherein the motion signature characterizes a movement experienced by the wearable fitness monitor; (b) obtaining, by the one or more processors, a heartbeat waveform signature obtained using data from the one or more heartbeat waveform sensors, wherein the heartbeat waveform signature characterizes a detected heartbeat waveform of a wearer of the wearable fitness monitor; (c) comparing, by the one or more processors, the motion signature and the heartbeat waveform signature or a combination thereof to one or more reference features of a user; and (d) determining, based on the comparison in (c) and by the one or more processors, whether an identity of the wearer of the wearable fitness monitor is the user.