Today, high-performance liquid chromatography is used to measure the actual capsaicinoid content of chiles in parts per million, which is then converted to Scoville Units.
Where does the heat come from in chiles? Capsaicinoids — tasteless, odorless compounds that are insoluble in water — can cause a "rapid bite" at the back of the palate and throat or a long, slow burn on the tongue.
Peppers produce these compounds only in glands located in their pods' placental partition, which is the white, fleshy membrane where the seeds are attached. The seeds become pungent through contact with the placenta but do not contain capsaicinoids of their own. If you want less heat, carefully remove the seeds and placental membrane before you use the peppers.
So why do chile peppers produce these capsaicinoids? When mammals eat peppers, they digest the seeds, destroying them. When birds eat peppers, they pass the seeds intact. Mammals can taste capsaicinoids, so peppers' heat may have evolved as a way to protect the seeds from mammals and allow birds to eat — and spread — them.
Scoville Heat Units (SHUs), developed in 1912 by Wilbur Scoville, are the standard industry method of measuring capsaicinoid levels. Originally, a precise amount of the chile was soaked in alcohol to dissolve the capsaicinoids. This, in turn, was dissolved in sugar water until a panel of testers no longer could detect any heat. At least three out of five testers had to agree on the dilution level. The SHUs reflect the number of units of sugar water it took to reach that point.
Today, high-performance liquid chromatography is used to measure the actual capsaicinoid content in parts per million, which is then converted to Scoville Units. Use any measuring system only as a guide because heat content varies by growing conditions, age, pod condition (fresh versus dried, for instance), and individual reactions to capsaicinoids.