This chapter introduces Symbiodiniaceae, the diverse group of dinoflagellate microalgae, that form an obligate symbiosis with corals and other coral reef organisms. The Symbiodiniaceae cells reside within the coral tissue, their photosynthesis fuels the productivity and diversity of coral reef ecosystem, and the breakdown of this symbiosis leads to coral bleaching and may entail the death of the host. Here, we summarize Symbiodiniaceae taxonomy and phylogeny and the molecular tools that are used to study Symbiodiniaceae diversity in the Red Sea. We provide an overview over all described Symbiodiniaceae species and discuss the functional diversity within this phylogenetically diverse group as well as the implications of this diversity for coral-Symbiodiniaceae pairings and ecological niche partitioning in coral reef ecosystems. We review host-Symbiodiniaceae associations of 57 host genera in the Red Sea and discuss the emerging patterns in light of their wider biogeographic distribution. Last, we summarize how climate change-induced thermal anomalies have repeatedly led to coral bleaching and mortality in the Red Sea and how they threaten these reef ecosystems, otherwise thought to be comparatively resilient. We conclude with a perspective of important topics for Symbiodiniaceae research in the Red Sea that have the potential to contribute to a broader understanding of the basis of thermotolerance in this fragile symbiosis.