This peer-reviewed resource from Bioscience Journal investigates the use of cell walls to shed light on the phylogenetic relationships. Cell walls have evolved independently in many phyletically diverse clades, including the Eubacteria and Archaebacteria. However, a review of the available genetic and biochemical evidence indicates that the machinery responsible for synthesizing the cell walls of land plants, of their nearest algal relatives (the charophytes), and of some very ancient algal lineages (chlorophytes, rhodophytes, and phaeophytes) can be traced back to ancient primary endosymbiotic events involving Eubacteria (specifically cyanobacteria and proteobacteria). Lateral gene transfers attending secondary endosymbiotic events appear to be responsible for manufacturing the cell walls of more recently evolved photoautotrophic and heterotrophic lineages (e.g., euglenoids and tunicates). Recent research into the genetic basis of cell wall synthesis and chemical composition in bacteria, algae, and land plants continues to shed light on the phylogenetic relationships among a broad spectrum of evolutionarily and ecologically diverse organisms.