The idea to produce this book originated during the "Tectonics and Sedimentation of Indonesia" seminar, the first regional meeting of the Indonesian Sedimentologists Forum (FOSI), the sedimentological commission of IAGI, in 1999. The meeting was conducted to commemorate the 50 years anniversary of the publication of van Bemmelen's book, entitled "Geology of Indonesia" (1949). This was the first book to deal indepth with Indonesian geology at that time. This work was written before the offshore exploration activities and is now out of print. Van Bem-melen's book is a classic of its type that covers onshore geology and some fields of geological en-quiry that have not been superceded by more modern work. In this volume, we have attempted to compile pre-existing publications. Additional recent insights understandings were added to give an up-to-date picture of the geology of Indonesia. It was found that much of geological data have been gathered since the first exploration activity some 150 years ago but was not utilized or even widely known by geologists recently. "An Outline of the Geology of Indonesia" is a compilation of mainly published materials using post van Bemmelen's concept and understanding. We have incorporated facts, ideas, philosophies, and prejudices of many authors; some are quoted and acknowledged; others have been absorbed and blended. These materials have been bound together to gave readers a regional view of Indonesia. Our main problems have been the selection of material and the need to strike a balance between each chapter. The book is far from perfect and ongoing efforts will be made to upgrade the contents of this book. Suggestions and input are most welcome to improve the next edition. The book is organized geographically into fourteen chapters, starting from the West and ended with Irian Jaya. Each chapter covers general geology, stratigraphy, and tectonic history. We have also endeavored to list as many references as possible in order to enable users to explore for themselves the original works.