Mycoplasma genitalium is a fastidious organism of the class Mollicutes, the smallest prokaryote capable of independent replication. First isolated in 1981, much is still unknown regarding its natural history in untreated infection. It is recognized as a sexually transmitted pathogen causing acute and chronic non-gonococcal urethritis (NGU) in men, with a growing body of evidence to suggest it also causes cervicitis and pelvic inflammatory disease in women. Its role in several other clinical syndromes is uncertain. The majority of people infected remain asymptomatic and clear infection without developing disease; asymptomatic screening is therefore not recommended. Prevalence rates are higher in patients attending sexual health clinics and in men with NGU. Limited availability of diagnostics has encouraged syndromic management, resulting in widespread antimicrobial resistance and given that few antimicrobial classes have activity against M. genitalium, there is significant concern regarding the emergence of untreatable strains. There is a need for wider availability of testing, which should include detection of macrolide resistance mediating mutations. Expertise in interpretation of microbiological results with clinical correlation ensures targeted treatment avoiding unnecessary antibiotic exposure. Public health surveillance nationally and internationally is vital in monitoring and responding to changing epidemiology trends. In this review, we summarize current knowledge of M. genitalium, including epidemiology, clinical and microbiological data, and discuss treatment challenges in the era of rising multidrug resistance.