Mycoplasma pneumoniae is a minimal microbe with respect to cell envelope composition, biosynthetic and regulatory capabilities and genome size, yet it possesses a remarkably complex, multifunctional terminal organelle. This membrane-bound extension of the mycoplasma cell is defined by the presence of an electron-dense core that appears as paired, parallel bars oriented longitudinally and enlarging at the distal end to form a terminal button. Most non-cytadhering mutants of M. pneumoniae isolated to date exhibit defects in the architecture of the terminal organelle. Detailed characterization of those mutants has revealed the identities of many component proteins of the terminal organelle as well as the likely order in which some of those components are required. Additional questions regarding the composition of the electron-dense core, the means by which the terminal organelle is duplicated during cell division and the manner in which this process is regulated remain to be answered. Thus, it seems that there is much to be learned about cellular engineering and spatial regulation in these 'simple' cell wall-less bacteria.