Since its designation as a separate genus some 30 years ago, Spiroplasmas have been well documented in a wide range of hosts and as the causative agent of several plant and insect diseases. One major area of research is the continued identification and taxonomical characterization of new Spiroplasma sp. combined with a determination of phylogenetic relationships among the various Spiroplasma sp. and between the Spiroplasmas and other members of the Mollicutes and Eubacteria. Although most phylogenetic analyses have been dependent on 16S rDNA sequence data, progress in two Spiroplasma sp. genome sequencing projects will provide new genomic regions for comparative focus. The co-evolution of Spiroplasmas with their arthropod hosts has provided an additional research focus to study host specificity and attachment. The diversity of symbiotic relationships between Spiroplasmas and their hosts has led to the study of commensal, mutualistic, and pathogenic relationships. Pathogenesis in insect hosts or in plants, transferred by insect hosts, is a major research focus, which requires attachment and invasion into insect tissues beyond the initial infection site, and successful movement to other tissues. The diversity and adaptations that have occurred during the evolution of the Spiroplasmas with their hosts will be the primary focus of this article.