Forensic toxicology is also used in employment drug testing, rehabilitation settings, and in human performance testing. The detection of drugs of abuse in potential employees prior to being hired is becoming an important application of toxicology. Attempts to exclude drugs from prisons and to aid in rehabilitation of drug-dependent persons are other applications of toxicology. Human performance testing relates to the detection of drugs that might have improved performance in athletic events. This testing may even apply to racing animals such as horses, camels, dogs, etc. Specimens used in these cases are usually urine, although hair is being increasingly used to provide a greater window of opportunity in workplace settings. In toxicology, we study both the adverse effects of chemicals on health and the conditions under which those effects occur. A natural outgrowth of biology and chemistry, toxicology began to assume a well-defined shape in just the past five to six decades. Newer still, environmental toxicology is concerned with the effects of chemical contaminants on various ecological systems, both large and small. Here, we focus on some of the basic principles of toxicology as related to environmental contaminants.