Excess fluid pressures exert important controls on the stability of lava domes and the flanks of volcanoes. Migrating overpressures reduce the shear strength of the edifice and may control the timing, morphology, and energetics of failure. Excess pressures may be developed both directly from magma degassing, and indirectly from the interaction of magma with infiltrating rainwater or groundwater. Interior gases influence the strength of the volcanic pile, and hence its stability, in at least twoways: (1) In the fractured and solidified outer carapace, high gas contents reduce effective stresses and concomitantly lower shear strength, and (2) In the dome interior, magmas that avoid off-gassing of volatiles exhibit a low strength, dominated by cohesion. Signatures of these various processes are evident in the extensive record of collapses that chart episodic growth and destruction of the lava dome at Soufrière Hills volcano, Montserrat. Mechanisms include interior pressurization by magma degassing, and the interaction of rainwater with the hot dome rind. The influence of gas overpressures applied interior to a brittle carapace is typified by the response to episodes of cyclic inflation, where, in addition to delays in the expected timing of failure, collapse may be triggered at inferred pressures below the peak obtained in the prior cycle. Similar influences on timing and collapse style are present for rainfall-triggered eventswhere deluges beyond a given intensity and duration are required to promote failure, and the style of collapse is influenced by antecedent conditions of gas pressurization within the lava dome. In all instances, interior gas overpressures or the presence of a segregated plastic core are both viable mechanisms to promote a switch between shallow instability of the dome carapace to deep transaction of the dome core. Such switching to a more hazardous and mobile failure mode may occur absent the usual seismic, geodetic, or chemical signatures that herald a collapse event, and presents special challenges in monitoring for hazard assessment.