Birth-Death (Bi-De) is an application for the Apple Macintosh which simulates the growth of phylogenetic trees using various models of lineage birth and death. The trees produced are intended to be analogous to those reconstructed from molecular sequence data. The user may define a constant birth rate and death rate or a function describing how these rates vary by time or population size. Instantaneous mass extinctions can also be simulated. The package allows the tree produced to be used as a template for the simulated evolution of molecular sequence data under a range of different transition models.