The morphology of the sclerotised components of the ovipositor system is comprehensively surveyed for scelionid and platygastrid wasps, with information being assessed for 120 genera and 220 species. A diagnosis for the ovipositor system is presented for most genera to complement existing generic descriptions. Two previously described and mechanically different forms of the ovipositor system are recognised: (1) the Ceratobaeus-type that is extended and retracted by antagonistic muscles and (2) the Scelio-type that is operated by changes in hydrostatic pressure, where the ovipositor is extended at the end of an elongate telescopic tube derived from expanded intersegmental membrane between metasomal segments 6 and 7. Comparison of these forms with the supposed ground plan for the Scelionidae strongly indicates that the Scelio-type is apomorphic, that it defines a monophyletic group associated with orthopteran host eggs, and that it comprises the tribes Scelionini, Calliscelionini, most Psilanteridini, Aradophagini, Neoscelionini, Platyscelionini, Doddiellini and four genera misplaced within the Sparasionini and Baryconini (Archaeoteleia Masner, Bracalba Dodd, Chromoteleia Ashmead and Oxyscelio Kieffer), as well as Sceliacanthella Dodd. Until a more robust classification of the superfamily is forthcoming, it is proposed that this group be informally referred to as the 'Scelionini sensu lato'. Further, seven genera (Habroteleia Kieffer, Palpoteleia Kieffer, Anteris Foerster, Fusicornia Risbec, Leptoteleia Kieffer, Opisthacantha Ashmead and Styloteleia Kieffer) are misplaced in the Calliscelionini and Psilanteridini because they possess the Ceratobaeus-type system. Nixonia Masner, Sparasion Latreille and Sceliomorpha Ashmead (Sparasionini) are considered to have the most primitive ovipositor system because they possess a Ceratobaeus-type system, and sub-basally fused lateral and latero-ventral apodemes, the latter being loosely attached to sternite 6. Sparasion and Sceliomorpha also have very short lateral apodemes and this, in conjunction with the form of the apodemes, can be considered to be the ground plan for the superfamily. The Platygastridae all possess a modified ovipositor system but, nonetheless, one that in most cases is extended and retracted by musculature (i.e. Ceratobaeus-type). In particular, the system in most platygastrids is typified by having metasomal tergite 8 and associated cerci missing, the lateral apodemes short and forming a U-shape, and the ovipositor assembly generally robust. Only one of approximately 30 genera examined, Acerotella Masner, has very elongate apodemes, as in the Scelionidae. Many platygastrids also have a pair of latero-ventral apodemes, a presumed plesiomorphic character, rather than a single medial apodeme on stemite 6, which is the case for many Scelionidae. The most highly modified system is found in Isostasius Foerster and some Synopeas (Sactogaster) Foerster, where the ovipositor assembly is coiled vertically or partly so and the apodemes are greatly reduced. Generally, characters associated with the ovipositor system do not provide any independent support for the most recent higher-level classification of platygastrids, although they show substantial potential for more accurate definition of genera. A preliminary cladistic analysis of 14 ovipositor characters supports the monophyly of five clades that correspond to the Scelionini s. l., the Scelionidae (minus the Sparasionini sensu stricto), the Sparasionini s. str., the Platygastridae, and the Sparasioriini s. str. + Platygastridae. Overall, results from this study will provide baseline information on the ovipositor system as a prelude to a more complete phylogenetic analysis of the superfamily including external morphological characters. Although no new classification for the Scelionidae and Platygastridae is proposed, their higher-level taxonomy is reviewed and discussed and cases identified where, on the basis of ovipositor morphology, taxa (tribes and/or genera) apparently form monophyletic groups, and where taxa are misplaced. Finally, the status of the major higher-level groups within the superfamily is discussed, as is the available evidence to support their monophyly.