The bacterial envelope is a complex, multi-layered structure, whose primary function is to protect the cell from environmental threats. For over a century, the Gram-staining method has been a foundational technique for classifying bacteria, distinguishing between Gram-positive bacteria, which retain the stain, and Gram-negative bacteria, which do not. Historically, prokaryotes have been classified in 41 accepted phyla of Bacteria and 16 phyla of Archaea, a system that overlooks the nuances revealed by Gram-staining properties. Advances in genomic sequencing have significantly reshaped our understanding of bacterial phylogeny. Additionally, insights into the genetics of cell envelope biogenesis have provided a new perspective on the bacterial Tree of Life. Bacterial envelopes are now categorized based on the number of membranes they possess, classifying cells as either diderms or monoderms. The presence or absence of lipopolysaccharides (LPS) in diderm envelopes is a critical distinguishing factor. This chapter provides a concise overview of the current knowledge regarding the composition and architecture of bacterial envelopes. It highlights how diverse structural adaptations serve a common purpose: ensuring the survival and protection of bacteria in various environments.