Thallium is a toxic and quite common element in the environment, but it is poorly examined, despite the known toxic activity of this metal. The ionic radius of the element is similar to the ionic radius of potassium, and at the same valence (K+,Tl+) allows substitution of this element in the nervous system and muscle, which results in its toxic activity. Thallium interferes with many enzymes and also has a strong affinity to the group – SH. The most dangerous symptoms of poisoning come from central nervous system. This element is easily cumulated by many living organisms, which poses a serious threat. In the past, it was used in rodenticides, and despite the risks, in many countries is still used for this purpose due to its low production costs. Contamination of this element of industrial areas is associated with the steel industry and cement production, while residents of these areas are often not aware of the threat and grow in these area vegetables that can absorb thallium, and in particular here talking about the Brassicaceae family. This element in the environment should be constantly monitored. Its isotope such as 201Tl (Thallium-201) has some significance in medicine, as radiopharmaceutical.