The normal karyotype of Caenorhabditis elegans, with its five pairs of autosomes and single pair of X chromosomes, is described. General features of chromosomes and global differences between different chromosomal regions are discussed. Abnormal karyotypes, including duplications, deficiencies, inversions, translocations and chromosome fusions are reviewed. The effects of varying ploidy and of varying gene dosage are summarized. Dosage-sensitive genes seem to be rare in C. elegans, and the organism is able to tolerate substantial levels of aneuploidy. However, autosomal hemizygosity for more than about 3% of the total genome may be incompatible with viability.