Phytophthora root and crown rot has been traditionally considered a minor disease of olive. However, in recent years it has been recognized as an emerging problem in several olive-growing countries such as Australia, Italy and Spain probably as a consequence of the expansion of plantings in new areas with heavy soils and the more intensive use of irrigation in both olive nurseries and commercial groves. The disease has been reported from most olive-growing countries and is caused by several soil-borne species of Phytophthora, including P. cinnamomi, P. citricola, P. cryptogea, P. drechsleri, P. gonapodyides, P. inundata, P. megasperma, P. nicotianae and P. palmivora. Diagnosis is currently based on the isolation and identification of isolates by both traditional and molecular methods. New molecular techniques are currently available that could be applied for both the identification of isolates and Phytophthora infections directly in host-tissues as well as in soil and water samples. A number of dedicated databases could improve the efficiency of these techniques. Moreover, DNA analysis has greatly contributed to phylogenetic studies of Phytophthora. Control of Phytophthora root and crown rot of olive is mainly based on preventive measures.