A copepod order of crustaceans containing 122 species in five genera, Cymbasoma, Guanabaraenia, Monstrilla, Monstrillopsis, and Thaumatohessia; Monstrillidae is the only family. Previously, the five species of the family Thaumatopsyllidae had been included as a second family of Monstrilloida, but in 2003 this family was removed to its own order, Thaumatopsylloida, and unpublished observations of the development support this decision.