Cover photograph (Copyright © 2003, American Society for Microbiology. All Rights Reserved.): Bacteria growing in biofilms often form multicellular, three-dimensional structures known as microcolonies. Cell death can occur inside Pseudomonas aeruginosa microcolonies, as illustrated in a horizontal cross section of a microcolony labeled with a fluorescent stain that differentiates between living (green) and dead (red) bacterial cells. Cell death may benefit a subpopulation of surviving cells within the biofilm and play an important role in subsequent biofilm differentiation and development. (See related article on page 4585.)