Toxicology is the study of the adverse effects of chemicals on living organisms. This chapter discusses the basics of toxicology, the major diseases, and associated toxins. When discussing poisons and toxins and their effects on humans or animals, one of the first things to consider is their method of entry into the body. The chapter discusses inhalation, ingestion, absorption, and percutaneous and intravenous routes of entry. The respiratory system consists of the nose, nasal passages, mouth, epiglottis, pharynx, larynx, trachea, bronchi, and alveoli. Percutaneous and intravenous injections are primarily medical procedures that invade the skin intentionally in order to administer a regulated dose of medicine. The dose–response relationship, or exposure–response relationship, describes the change in effect on an organism caused by differing levels of exposure to a stressor after a certain exposure time. There are four stages of cancer. They are: initiation, latency period, promotion, and progression.