SummaryThe architecture of eukaryotic cells is underpinned by complex arrrays of microtubules that stem from an organizing center, referred to as the MTOC. With few exceptions, MTOCs consist of two basal bodies that anchor flagellar axonemes and different configurations of microtubular roots. Variations in the structure of this cytoskeletal system, also referred to as the ‘flagellar apparatus’, reflect phylogenetic relationships and provide compelling evidence for inferring the overall tree of eukaryotes. However, reconstructions and subsequent comparisons of the flagellar apparatus are challenging, because these studies require sophisticated microscopy, spatial reasoning and detailed terminology. In an attempt to understand the unifying features of MTOCs and broad patterns of cytoskeletal homology across the tree of eukaryotes, we present a comprehensive overview of the eukaryotic flagellar apparatus within a modern molecular phylogenetic context. Specifically, we used the known cytoskeletal diversity within major groups of eukaryotes to infer the unifying features (ancestral states) for the flagellar apparatus in the Plantae, Opisthokonta, Amoebozoa, Stramenopiles, Alveolata, Rhizaria, Excavata, Cryptophyta, Haptophyta, Apusozoa, Breviata and Collodictyonidae. We then mapped these data onto the tree of eukaryotes in order to trace broad patterns of trait changes during the evolutionary history of the flagellar apparatus. This synthesis suggests that: (i) the most recent ancestor of all eukaryotes already had a complex flagellar apparatus, (ii) homologous traits associated with the flagellar apparatus have a punctate distribution across the tree of eukaryotes, and (iii) streamlining (trait losses) of the ancestral flagellar apparatus occurred several times independently in eukaryotes.