In plant breeding and population genetics, computer simulations are a useful tool to investigate problems for which no analytical solutions are available. We present PLABSIM, a computer program to simulate marker-assisted selection in arbitrarily designed backcross programs. The simulated data can be evaluated with PLABSIM for gene frequencies, genotype frequencies, frequency of homozygous loci, length of chromosome segments originating from one ancestor, and the number of marker data points required for a breeding program. In addition to data analysis with PLABSIM, the simulated data can be exported for analysis with statistical software. For investigations of the potential applications of marker-assisted selection, computer simulations are a useful tool. They have been used to investigate problems concerning the use of flanking markers in selection for one or more target genes (Hospital and Charcosset 1997; Visscher et al. 1996) or to accelerate the recovery of the recurrent parent genome in markerassisted backcross programs (Frisch et al. 1999; Hospital et al. 1992; Openshaw et al. 1994). Following Hospital and Charcosset (1997), we refer to the first approach as foreground selection and to the second as background selection. Until now, no software allowed one to simulate marker-assisted selection under realistic genetic models. The program GREGOR (Tinker and Mather 1993) implements the basic principles, but the interactive use and the fact that it simulates only some predefined genetic linkage maps restricts its feasibility for simulation of breeding programs. In this article we present PLABSIM, a tool for simulation of marker-assisted selection programs. The software can be used to investigate the effect of varying population size, marker density, marker positions, and selection strategies on the genetic composition of the breeding product and on the required number of marker data points. PLABSIM is characterized by the following features: (1) Simulations can be made for any diploid genome with an arbitrary number of loci at arbitrary positions on an arbitrary number of chromosomes. (2) The implemented reproduction schemes include all common breeding methods. (3) An arbitrary number of selection steps can be combined to a selection strategy. Selection can be carried out for genotypes at defined loci, or for selection indices calculated from allele frequencies at several loci. (4) The simulated data can be analyzed for a broad range of genetic parameters with PLABSIM. In addition, the data can be exported for analysis with statistical software.