The TEX86 paleothermometer is based upon the distribution of archaeal membrane lipids ('GDGTs') in marine sediments. GDGTs are ubiquitous, abundant and relatively resistant to degradation; as such, the TEX86 paleothermometer has been used to reconstruct sea surface temperature (SST) during the Cenozoic and early Mesozoic. We review the principles of the TEX86 proxy and developments made over the last two decades. We also discuss its application as a paleotemperature proxy and explore existing challenges and limitations.