Knowledge gaps in taxonomy and systematics are present in all insect groups, differing just in size. Generally, size of the gap is negatively correlated with body size and positively with number of species within the group. As an example, we present the size of the knowledge gap within the subfamily Aphidiinae (Hymenoptera: Braconidae). As solitary koinobiont endoparasitoids of aphids, Aphidiinae are widely used as biocontrol agents and consequently one of the best studied parasitoid groups. In the last decade, taxonomical studies on Aphidiinae have been focused on an integrative approach which resulted in numerous new findings: description of new species, detection of alien species, resolving the taxonomical status of genera and species groups, proposition of new classification schemes, etc. All those findings are critically summarized. Availability of different types of data is discussed and propositions for future studies and optimal research practices are given.