Seismic data recorded on Iliamna Volcano, Alaska, indicate that Iliamna ice avalanches are preceded by up to several hours of precursory seismicity. This precursory activity is represented by a 1-3 hour period of activity comprising a sequence of discrete repeating earthquakes and a period of continuous groundshaking that builds in amplitude. The avalanche itself is represented by a broadband (1-20 Hz), spindle-shaped signal that may be composed of several pulses of activity. The precursory earthquakes are similar in time series, indicating that they share a common source region and mechanism, and become more and more frequent until they blend into the continuous signal. These events are interpreted as slip along the base of an ice layer. The continuous groundshaking is believed to represent the failure of small slip zones beneath the ice slab. Slip becomes larger as the event progresses, until the slab fails in a catastrophic avalanche. Estimates of lateral runout distance and signal duration yield velocities consistent with avalanche velocities recorded elsewhere. The fact that these events exhibit precursory seismicity while other debris avalanches and rockfalls, including one that occurred on Iliamna in May 2005, do not may result from the fact that the initial failure took place in ice. The prolonged duration of the precursory phase suggest s that some warning may be possible for similar events in the future.