For most of Earth history the only macroscopic evidence of life are the intricately layered rocks called stromatolites. Like trace fossils they record an interaction between organisms and sedimentary processes. The key players are the cyanobacteria, formerly known as blue-green algae, which played a vital role in oxygenating the Earth through their photosynthesis. Build-up of sediments and precipitates over time under the sticky surface mats created by communities of such organisms mats can make limestone and dolomite masses up to hundreds of metres thick. Stromatolites reveal many fascinating stories of past environments and examples from several continents will be discussed.