Research into the mechanisms responsible for the lasting, cyclic activity at Masaya volcano can lead to a better understanding of persistently degassing volcanoes. This study is greatly enhanced by the integration of dynamic micro-gravity, deformation and gas flux measurements. The acquisition of extended temporal and spatial geophysical data will also allow for the development of robust models for the dynamics of magmatic systems. Masaya volcano, Nicaragua, is one of the most active systems in Central America, making it an excellent natural laboratory for this study. It is noted for repeated episodes of lava lake formation, strong degassing and subsequent quiescence. Ground-based geophysical measurements show two episodes of similar magnitude gravity decreases in 1993-1994 and 1997-1999, separated by a period of minor gravity increase. A major increase in S02 gas flux from 1997-1999 correlates well with the most recent episode of gravity decrease. The gravity changes are not accompanied by deformation in the summit areas and are interpreted in terms of sub-surface density changes. The persistent degassing at Masaya suggests that up to -15 krrr' of magma may have degassed over the last 150 years, only a minute fraction of which has been erupted. Furthermore, thermal flux calculations suggest that 0.5 krrr' of magma (the estimated volume of the shallow reservoir) would cool from liquidus to just above solidus temperatures in only 5 years. The high rates of degassing and cooling at open-system volcanoes such as Masaya raise questions as to the ultimate fate of this degassed and cooled magma. A number of models have been proposed to explain this, but the most likely mechanism to explain persistent activity at Masaya and other similar volcanoes is convective removal of cooled and degassed magma and subsequent recharge by volatile-rich magma from depth. Another fundamental question in modem volcanology concerns the manner in which a volcanic eruption is triggered; the intrusion of fresh magma into a reservoir is thought to be a key component. The amount by which previously ponded reservoir magma interacts with a newly intruded magma will determine the nature and rate of eruption as well as the chemistry of erupted lavas and shallow dykes. The physics of this interaction can be investigated through a conventional monitoring procedure that incorporates the Mogi model relating ground deformation (~) to changes in volume of a magma reservoir. Gravity changes (.1.g)combined with ground deformation provides information on magma reservoir mass changes. Models developed here predict how, during inflation, the observed .1.gI~ gradient will evolve as a volcano develops from a state of dormancy through unrest into a state of explosive activity.