In close connection with Melanesian and, later, European settlement in New Caledonia, exotic species of plants and animals were initially voluntarily introduced to fulfil practical and economic purposes. In the last decades however, the introduction of alien species has primarily been driven by leisure activities and hobbies. Eighty-three per cent of all vertebrates introduced since 1950, in particular since 1970, have been brought in to satisfy the demand for hunting, angling, and the aquarium and cage bird trade. Such voluntary introductions were inevitably accompanied by numerous accidental ones. There are about 800 exotic species of flowering plants, at least 400 invertebrates and 36 vertebrates now established in the wild in New Caledonia. In combination with fire and clearing, vast areas of native vegetation, in particular of sclerophyll forest, have been replaced by secondary savanna and scrubland dominated by a few dozen invasive species of plants and animals. As increasing extents of ''natural'' habitats are being transformed through this process, an effective control of introductions should be established and enforced. Indeed current legislation totally lacks ecological considerations. Extinction or endangerment of several endemic species can be traced to the presence of introduced mammals, and the authors highlight several cases of introductions which have led to such depletion of native New Caledonian biodiversity. Restoration of selected areas through eradication of exotic pests will probably be essential for the in situ conservation of the unique biodiversity of New Caledonia.