The application of nucleic acid probes and related techniques is becoming common in the detection of human and other animal viruses but has lagged somewhat in plant virology. The explosion in the use of the technology associated with recombinant DNA research has led to the development of various techniques by which viruses can be diagnosed. Many of these are based upon the hybridization between the target viral nucleic acid, which in most cases is immobilized onto a solid matrix, and a probe nucleic acid. Although most of plant virus diagnosis now involves mixed-phase hybridization with the target immobilized on a solid matrix, the theory developed for liquid-liquid hybridization is very relevant. The kinetics of mixed-phase hybridization have been less well studied than those of liquid-liquid hybridization. Liquid-liquid hybridization has been used to examine the relationships between plant viruses. Most of the approaches to the use of nucleic acid hybridization for virus diagnosis involve the use of mixed-phase hybridization.