Discharges from a volcano such as volcanic ejecta, ashes, gases and hot spring waters can provide us with a variety of underground information of the volcano, since the change in volcanic activity brings about the changes in chemical compositions of the volcanic exhalations. In the present paper we have reviewed the chemical anomalies in the volcanic gases which are related to volcanic eruptions. It is common that the chemical anomalies observed at a different type of volcanic eruptions are very different, but they also seem highly variable even for the eruptions of a similar type. Although an overwhelming majority of the anomalies were recognized as “precursors” after the eruptions, accumulation of such information is quite important to give basis to the geochemical prediction of volcanic eruptions, since our knowledge is still highly limited. The chemical anomalies observed so far have mostly been obtained by repeated spot analyses with variable time intervals. This often makes it difficult to determine when the anomalies did start and how long they lasted. Therefore, we should perform truly continuous observation and collect as much geochemical and geophysical information as possible on each active volcano in question in order to make the geochemical prediction of the change in volcanic activity possible with confidence.