Lettuce mosaic virus (LMV) is an economically significant virus of lettuce and endive. The virus has spread world‐wide due to the exchange of seed of lettuce varieties. A very high proportion of infected plants may result from a low level of infected seed, because of very efficient transmission by a number of aphid species. The symptoms are characteristic but the diagnosis can be difficult, particularly on lettuce, because numerous viruses may coinfect this species. A very reliable and sensitive method by ELISA has been established for diagnosis and detection, which gives a good estimation ofthe contamination level in a seed batch. The use of virus‐free seed, preventive cultural practices and the use of tolerant varieties were shown to be good methods for control if rigorously applied. Up to now, strains able to overcome the genes g and mo, considered to be identical, were shown to be non‐seed‐transmissible. Studies carried out with several virulent isolates have shown that genes mo and g are different and probably allelic, and that one strain infects seed at a very high level on susceptible and tolerant genotypes. These features have necessitated the production of virus‐free seed, including systematic checks on all cultivars, and have stimulated research on new sources of resistance. Recent molecular studies have provided clones for detection and strain differentiation. Assays to introduce different LMV genes into lettuce seem promising.