The simplest carbon-based functional groups are alkenes (olefins) and the much rarer alkynes, containing only C–H and C–C bonds but no C–O, C–N, or C–S bonds. The biologic routes to both trans- and cis-alkenes are examined. The reactivity of olefins as either electron rich carbon nucleophiles or as electrophilic, electron poor carbon sinks depends on the structural and electronic context of the olefins and their partner reactants. The ability of 2-isopentenyl-PP to act as progenitor to an electrophilic allyl cation and 3-IPP to act as an olefinic nucleophile is the fundamental chemical logic for C–C bond formation in isoprenoid chain extension reactions. Epoxidation of olefins and nucleophilic addition to conjugated olefins reveal nucleophilic vs. electrophilic reactivity, respectively.