Ciliates are the most complex of cells, having an elaborate cytoskeleton, cilia and two different kinds of nuclei. Free‐living ciliates can be found in almost any habitat that has water – in soils, hot springs and Antarctic sea ice. Symbiotic species live as commensals in sea urchins or as parasites of lobsters and fish. Ciliate life histories can have specialised forms for dispersal and for resisting desiccation. Their cell cortex is supported by a complex framework of basal bodies or kinetosomes, microtubules and microfilaments. The kinetosomes form the central unit in an organellar structure called the kinetid, which is important to understanding phylogenetic relationships among ciliates. The pattern of fibres and microtubules in the kinetid identifies a ciliate to a major clade, along with the sequences of genes. These two features together identify 12 major clades or classes of ciliates.