Sentinel species are organisms, often animals, used to detect risks to humans by providing advance warning of a danger. The terms primarily apply in the context of environmental hazards rather than those from other sources. Some animals can act as sentinels because they may be more susceptible or have greater exposure to a particular hazard than humans in the same environment. People have long observed animals for signs of impending hazards or evidence of environmental threats. Plants and other living organisms have also been used for these purposes.

Many observations of animals point to toxicity in food, water or air that would or could harm humans.

The classic example is the "canary in the coal mine". The idea of placing a warm-blooded animal in a mine to detect carbon monoxide was first proposed by John Scott Haldane in 1895, and canaries were used as early as 1896. Countries such as Britain, the United States, and Canada used canaries as a sentinel species. Well into the 20th century, coal miners brought canaries into coal mines as an early-warning signal for toxic gases, primarily carbon monoxide. The birds, being more sensitive, would become sick before the miners, who would then have a chance to escape or put on protective respirators. In some cases, the canaries were kept in cages with dedicated oxygen tanks so the birds could survive after their illness provided a warning. "Canary in the coal mine" is now used as an idiom for a person or thing that warns people of danger.

In Minamata Bay, Japan, cats developed "dancing cat fever" before humans were affected due to eating mercury-contaminated fish.

Dogs were recognized as early as 1939 to be more susceptible to tonsil cancer if they were kept in crowded urban environments. Studies similarly found higher disease rates in animals exposed to tobacco smoke.

In 1968, Yushō disease was discovered when poultry began dying at alarming rates due to polychlorinated biphenyl (PCB) poisoning, although not before approximately 14,000 people were affected.^{[citation needed]}

Rose bushes are often planted at the ends of vine rows in vineyards. One hypothesized reason is that roses are highly susceptible to powdery mildew, which can also afflict grape vines. Finding mildew on the roses is therefore sometimes taken as an early warning sign to take preventive measures to protect the adjacent vines. However the effectiveness of this practice is debated, since roses and vines are affected by different species of mildew (although both species occur under similar weather conditions).

In Poznań, Poland, eight mussels equipped with sensors are used to ensure the safety of the city's drinking water. These mussels act as bioindicators, meaning they provide a reliable estimation of water toxicity by responding to changes in water quality.