Cilia are ubiquitous, hair-like protrusions attached to cells. Interactions between cilia and ciliated tissues mediate a variety of physiological flows. Whenever multiple cilia exist in close proximity they will invariably interact, leading to the emergence of many types of local and global coordination patterns. Often, the mechanism of this interaction or coupling is mysterious and highly system-dependent. Adjacent cilia can communicate physically through the fluid, but they can also do so via elastic or cytoskeletal linkages through the cell or tissue surface. In this talk we will consider the strategies and consequences of distinct modes of ciliary coordination and propulsion in diverse organisms ranging from single-celled eukaryotes (protists), to the ciliated larvae of marine invertebrates. We will introduce different model systems in which groups of cilia can move completely synchronously, maintain specific synchronization patterns, or else beat metachronously along topologically interesting structures. We will also discuss how ciliary arrays select different modes of synchrony or metachrony, transitioning stochastically between order and disorder, and the implications of this for whole-organism self-propulsion and navigation in the vast depths of the ocean.