In the marine environment, invertebrates like corals, jellyfishes, or mollusks are well known by SCUBA divers as potential toxic species. Among marine invertebrate animals, the phylum Cnidaria is one of the oldest living groups. Marine cnidarians are mostly distributed among four classes: Anthozoa, Cubozoa, Scyphozoa, and Hydrozoa. Very potent cytolytic proteins have been isolated from jelly fish of both Scyphozoa and Cubozoa groups. Peptides and proteins constitute the largest group of toxins found in cnidarians. A large number of peptides with low molecular weights have been found in sea anemones. The most thoroughly characterized peptidic toxins are those affecting sodium and/or potassium channels. All classes of the phylum Cnidaria present catalytic activity of phospholipases A2 enzymes, suggesting that these enzymes can be associated to the toxicity of this animal group. Nonpeptidic toxins have been identified in the octocoral anthozoan branch, such as polyethers, steroids, acetogenins, sesquiterpenes, and a large number of terpenoids, especially diterpenoids, and compounds related to pigments.