The castor plant (Ricinus communis L.) has been known since time immemorial in traditional medicine in the pharmacopeia of Mediterranean and eastern ancient cultures. Moreover, it is still used in folk medicine worldwide. Castor bean has been mainly recommended as anti-inflammatory, anthelmintic, anti-bacterial, laxative, abortifacient, for wounds, ulcers, and many other indications. Many cases of human intoxication occurred accidentally or voluntarily with the ingestion of castor seeds or derivatives. Ricinus toxicity depends on several molecules, among them the most important is ricin, a protein belonging to the family of ribosome-inactivating proteins. Ricin is the most studied of this category of proteins and it is also known to the general public, having been used for biocrimes in several cases. Here, the main steps of ricin research are reported with particular regards to its enzymatic activity, structure and cytotoxicity. Moreover, we discuss ricin toxicity for animals and humans, as well as the relation amongst bioterrorism and ricin and its impact on environmental toxicity. Ricin has also been of great utility to develop a number of immunotoxins specific for the elimination of unwanted cells, mainly cancer cells; some of these immunotoxins gave promising results also in clinical trials.