Indonesia is one of the most active volcanic areas in the world and has a population of 150 million. Natural disasters affecting large areas and numbers of people are therefore a common occurrence and the authorities have developed strategies to cope with them. On 5 April 1982 Mt Galunggung in W Java suddenly erupted after 64 years and it is still active. From April to June this activity coincided with the rainy season; the volcanic mud and boulder flows (lahars) which were generated ended in late November. Tens of millions of cubic metres of unconsolidated volcanic ejecta, mostly ash and sand size, were deposited around the newly formed crater on the slopes of the volcano and over a wider area. In anticipation of the next rains check dams and other works were planned, as it was feared that as many as 250 000 people might be affected, and further large areas of agricultural land inundated as a result of rain generated lahar. A civil defence system has been initiated, utilizing the experience gained as result of previous similar disasters. The paper summarizes the geological background, and describes the related types of primary and secondary hazards and the attempts being made to mitigate the effects of these hazards.