Merapi volcano, located in Central Java, is one of the most active volcanoes in Indonesia. 2 million people are living in its immediate neighborhood. Therefore Merapi was selected within the International Decade of Natural Disaster Reduction (IDNDR) of UNESCO as one of 15 so called high risk volcanoes in the world. National and International research groups from Indonesia, France, Netherlands, Japan, USA and Germany are working on Merapi. Different methods are applied on Merapi to study the volcanic processes and to improve the possibilities to predict future eruptions. In this thesis the importance of gravity changes in space and time for the analysis of volcanic processes is analyzed and further developed. First the basic theory of Earth's gravity field and gravity anomalies is described. For the interpretation of gravity anomalies several programs have been developed using the MATLAB software package. The programs are used for the interpretation of gravity changes in time which have been observed five times between summer 1997 and summer 2000 in a repetition network around Merapi volcano. This network consists of 23 stations. During all campaigns four LaCoste&Romberg gravimeters model G and D have been used. Height changes at the observation sites are controlled by GPS-observations which have been carried out in parallel with the gravity measurements. The observed gravity and height changes are small. They reveal that Merapi volcano is just now an open system where no large stresses can build up. Nevertheless possible models are developed to explain the observed changes. Based on four geometric models of magma chamber and conduit within the volcano?s edifice as found in the literature the migration of magma in the conduit is investigated. The conduit system thereby is modeled by a cylinder, the magma chamber by a sphere. It is shown, that gravity changes map the migration of the magma, if gravity changes at stations at the crater rim are considered. Gravity changes at stations far away from the volcano conduit (2 ? 25 km) can be explained by changes within the geohydrothermal system. For this purpose particular ground water layers as determined by other geophysical methods (resistivity observations, magnetotelluric measurements and LOTEM) are modeled as concentric cylinders around the conduit of Merapi volcano. The resulting density changes in the cylinders explain with sufficient accuracy the observed gravity changes. Precise repeated gravity observations in combination with other geophysical methods allow therefore the detailed analysis of subsurface mass migration within a volcano.