Thermal bleaching tolerance of corals is partly dependent on the thermal tolerance of their microalgal endosymbionts (Symbiodiniaceae). While natural variation occurs in Symbiodiniaceae thermal tolerance, host compatibility can be limited to a small number of symbiont species and trait trade-offs with thermal tolerance exist. The in vitro enhancement of Symbiodiniaceae thermal tolerance via experimental evolution and subsequent reintroduction of the heat-evolved symbionts into corals may be used to boost fitness under continuing climate warming for a taxonomically diverse range of corals. In this presentation, recent developments in this field will be discussed including selection under fluctuating rather than flat-lined temperatures, and the use of chemical mutagenesis followed by thermal selection to increase the in vitro thermal tolerance of strains spanning the genera Symbiodinium, Cladocopium, Durusdinium and Fugacium. Experimental evolution resulted in improved physiological performances (growth rate, photosynthetic efficiency, ROS levels) of the heat-evolved strains under elevated temperature compared to their wild-type counterparts. Concurrently, we successfully introduced heat-evolved symbionts into chemically bleached adult corals, providing a methodology to assess the performance of adult corals that associate with heat-evolved Symbiodiniaceae. Our results provide important steps towards the incorporation of experimental evolution approaches into coral reef conservation and restoration initiatives.