Short-term prediction in volcanology i.e., the reliable estimation of (1) an appropriate time window during which an eruption can take place and (2) the kind and magnitude of the eruption expected, needs to be examined, focusing research interest on physical aspects of volcanic activity. The problem consists of deducing the internal structure of the volcanic complex and the dynamic properties of the fluids involved from the associated physical and physico-chemical phenomena observed during the pre-eruptive phase. Geological and petrological investigations, as well as the analysis of volcanic gases, represent substantial sources of information in accomplishing this task, but they are not substitutes for physical examination. To solve this problem, adequate observations and experiments should be performed. Today's geophysical approach in the investigation of volcanic activity allows at best the reconstruction of the internal structure of the volcanic complex. Methods to detect and to quantify the dynamics of the processes suspected of triggering eruptive activity are practically nonexistent. In solving the short-term prediction problem through the understanding of processes triggering volcanic eruptions, the estimation of the state of the fluid system represents a crucial point. In order to improve our knowledge of the dynamical properties of the underlying fluid system, the comprehension of seismic-source mechanisms is of paramount significance. Mechanisms generating some types of observed seismic activity and originating from variations in the fluid-conduit pressure field are an invaluable aid toward the reconstruction of the internal state of the volcano. Attention must be centered on the influence of a free gas phase in magma upon the generation of 'not-earthquake-like' ground vibrations in volcanic areas. A mixture composed of liquid containing gas bubbles (gas + liquid mixture) can likely cause large pressure fluctuations which, when transmitted to the confining rock structure, are radiated as seismic waves, the characteristics of which are carriers of the state of the fluid system. To fully estimate the state of the magma dynamics, detectable changes of non-seismic field variables must be considered as well.