Genetic suppression has provided a very powerful tool for analyzing C. elegans. Suppression experiments are facilitated by the ability to handle very large numbers of individuals and to apply powerful selections. Because the animal grows as a self-fertilizing diploid, both dominant and recessive suppressors can be recovered. Many different kinds of suppression have been reported. These are discussed by category, with examples, together with discussion of how suppressors can be used to interpret the underlying biology, and to enable further experimentation. Suppression phenomena can be divided into intragenic and extragenic classes, depending on whether the suppressor lies in the same gene as the starting mutation, or in a different gene. Intragenic types include same-site replacement, compensatory mutation, alteration in splicing, and reversion of dominant mutations by cis-knockout. Extragenic suppression can occur by a variety of informational mechanisms, such as alterations in splicing, translation or nonsense-mediated decay. In addition, extragenic suppression can occur by bypass, dosage effects, product interaction, or removal of toxic products. Within signaling pathways, suppression can occur by modulating the strength of signal transmission, or by epistatic interactions that can reveal the underlying regulatory hierarchies. In C. elegans biology, the processes of muscle development, vulva formation and sex determination have provided remarkably rich arenas for the investigation and exploitation of suppression.