Aquatic phototrophs have a remarkable ability to cope with variations in a range of environmental factors, such as light, temperature, pH and salinity. However, some environmental conditions are beyond what are considered the normal limits for growth and can thus be considered as extreme. Focusing on algae and cyanobacteria, we discuss the capacity of extremophilic organisms to cope and even thrive in extremes of temperature ranging from hot springs to snow and ice algae, under extremes of pH and in situations where water is in short supply, such as in biological soil crusts and on man-made surfaces such as buildings and statues. Many of the mechanisms that allow algae to cope with these extremes are common across different situations and involve, for instance, processes to dissipate excess light energy and deal with reactive oxygen species. Algae and cyanobacteria enter symbiotic associations with other organisms, such as lichens and corals. They are also found as intracellular symbionts in plants, other algae and various protists and metazoans. There are looser associations where algae grow on animals such as gastropods, seals and terrestrial animals such as sloths. We also discuss the retention of active chloroplasts by phagotrophs in the process of kleptoplasty.