Thischapter focuses on data from Czech, specifically, derived aggregates and complex numerals. Derived aggregates such as list-í (leaf- í- NOM.SG, ‘foliage’) are derived from a count noun (e.g., list) and the suffix -í. Furthermore, aggregates derived with -í are non-count. Grimm and Dočekal discuss three types of complex numerals that are also morphologically derived: derived group numerals, complex numerals for aggregates, and taxonomic numerals. As well as using these data to bolster the support for a view in which aggregates are a distinct kind of non-countable entity from substances, the authorsargue the complexity of the morphosyntactic resources a language can limit the extent to which nouns in that language are mass/count flexible. These data are then modeled in a neo-Krifkan framework, enriched with mereotopology that accounts for taxonomic and group numeral constructions, the sense in which derived aggregates denote clustered individuals (individuals formed of parts that stand in relatedness relations to one another), and why the counting of derived aggregates in Czech is only possible with complex numerals for aggregates.