Mycoplasma gallisepticum and M. synoviae are the most relevant mycoplasma species for commercial poultry from the clinical and economic point of view. Although the importance of M. gallisepticum was recognized many decades ago, the relevance of M. synoviae has been a matter of debate. Until the turn of the century, only the respiratory and synovitis forms of the disease were reported, while the majority of infections were subclinical. Since the year 2000 M. synoviae strains with oviduct tropism, able to induce eggshell apex abnormalities and egg drops, have been encountered worldwide. A decreasing incidence of M. gallisepticum has been observed, at least in breeding stock, in countries with control and eradication programmes for this Mycoplasma species. In contrast, the sero-prevalence of M. synoviae is much higher, especially in layer flocks, and in most continents exceeds 70%. Given the emergence of virulent M. synoviae strains with oviduct tropism, its ability to also induce joint and respiratory disease, to act synergistically with other pathogens as well as its much higher sero-prevalence, it seems that M. synoviae is outrunning M. gallisepticum, at least in countries with control and eradication programmes for the latter. This stresses the need to update M. synoviae prevention and control strategies. Thus, in January 2013, the Dutch poultry industry implemented a mandatory control and eradication programme for M. synoviae at all levels of poultry farming with the exception of broilers.