The primary photosymbionts of tropical reef-building corals belong to the microalgal family Symbiodiniaceae. These eukaryotic dinoflagellates, commonly known as ‘zooxanthellae,’ form intracellular associations with cnidarian hosts and represent a key nutritional component of the coral microbiome. The cnidarian-dinoflagellate mutualism fuels coral skeletal growth, generating the three-dimensional habitat that supports the immense diversity of reef-associated organisms and builds the foundation of reef ecosystems to provide ecological goods and services. To predict how coral reefs will respond in a warming world, it is vital to comprehensively describe the extant biodiversity underlying Symbiodiniaceae. Such a catalog serves as a foundation to understand the adaptive capacity of the algal symbionts, the coral hosts, and the combined holobiont. This overview chapter is targeted at microbiome researchers who may be new to the coral reef community and Symbiodiniaceae world (welcome!). It briefly covers several important aspects of symbiodiniacean biology: their energetic contribution to reefs, their astounding diversity, their role in coral bleaching, their molecular interactions with hosts, their evolutionary history, and their own microbial associations. It concludes with advice for interpreting past literature while developing new research.