Santiaguito volcano has shown a continuous slow extrusion of dacite lava since 1922. In the 50 years of activity there have been four periods of abnormally high extrusion rates, interspersed by periods of little magma production. The type of activity shown by the volcano has been varied and crudely cyclic. Dome extrusion periods are accompanied by pyroclastic activity and followed by lava flows. There are now 16 time stratigraphic units delineated on the dome. Activity since 1967 has been especially closely observed. Dome extrusion at the west end of the complex has been accompanied by pyroclastic cruptions and plug dome extrusion at the east end. The eurrent extrusion rate has remained essentially constant since 1967 at about 5×106 m3/yr, far below Santiaguito’s 1922–71 average of 14×106 m3/yr. The active vent at the east end of the volcano (Caliente vent) has been the principal vent of the volcano since the creation of the explosion crater in 1902. After its initial period of dome extrusion (1922–25), the Caliente vent has chiefly produced pyroclastic eruptions as well as at least 95% of the dome’s lumarolic activity, while lateral vents have continued to give rise to lavas. Lava flows at Santiaguito have effective viscosity values of about 106 poises, while dome lavas are significantly more viscous. The differences in viscosity are in part related to volatile content of the lava when it reaches the surface. During dome extrusion, lavas lose their volatiles through pyroclastic activity before they reach the surface. Lava flows at Santiaguito occur when lava reaches the surface with higher volatile content. Obstruction of either the central (pyroclastic) vent or the lateral (dome extrusion) vent or both vents has an important influence on succeeding activity. In June 1972, at the time of this writing, the outbreak of new lava flows at both the Caliente and lateral El Brujo vents has just occurred, resulting from obstruction of pyroclastic activity by a large plug dome at the Caliente vent.