Petrologists regard enclaves (mafic inclusions, xenoliths,...) as essential for understanding the genesis and evolution of igneous rocks, in particular, the petrology of granitoids. This volume is a collection of forty papers which represent an up to date inventory and an open discussion of the main concepts proposed for the origin and evolution of enclaves in igneous rocks. The contributors are from Australia, China, Japan, USA, USSR and Europe. In the first part of the volume, various types of enclaves are investigated in granitoids of different natures and ages, emplaced in contrasted tectonic environments. Some of these granitoids are from reference areas such as the Lachlan Fold Belt, Australia, the Massif Central, France, and the Sierra Nevada, California. Enclaves from volcanic rocks of Japan and France are also included. The second part begins with a discussion of the macroscopic and microscopic features, textures, mineralogy and chemistry (major, trace elements and isotopes) of enclaves and the host granitoids. Various and often contrasting models including the restite model are then proposed for the genesis and evolution of enclaves and the host granitoids. The favored magma mixing model is detailed through the full set of petrologic tools including laboratory experiments. The extensive bibliography included refers to most papers published on enclaves in the last twenty years.