Polyploidy (an increase in the number of whole chromosome sets) is defined, illustrated, and explained. Mechanisms of polyploidy are discussed and the effects are examined for any possible role in progressive evolution. Although polyploidy may introduce changes in size, osmotic pressure, fertility, and other factors, these same changes are often unfavorable to the polyploid and are not the kinds of changes required for major evolutionary steps. Aneuploidy (addition or subtraction of a single chromosome) is shown to be the basis of several disorders in the human body. Rather than adding extra genetic material from which to evolve new traits, aneuploidy is usually harmful to the species, and not creative in its effect. Rearrangements of chromosome parts (translocation and inversions) are also seen to be generally harmful. Although the various types of chromosomal changes may produce some striking modifications in the organism, they can hardly be regarded as important evolutionary mechanism.