The paper outlines the impact of molecular biology on the identification and detection of fungi and fungus-like micro-organisms. We have tried to use as little jargon as possible but avoiding it completely is very difficult. The article is centred around the ‘pseudofungus’ Phytophthora and its closest relatives, because that is the micro-organism with which we have had most experience. Nevertheless, most of the approaches and techniques mentioned are applicable to any fungus or fungus-like organism and, in that broader sense, the article should be of interest to those with no special interest in or knowledge of Phytophthora.