Very hot chiles that are smaller and hotter than jalapeños. Serranos are most often sold green, when they lend a tangy, herbal, vegetal flavor. In late summer, you may see fully ripe, red serrano chiles.
Substitute another small, hot chile, such as the Thai bird chile.
Choose chiles that are shiny and firm pods with strong, uniform color. They should feel dense and heavy for their size; good examples of even the very smallest ones will feel heavier. Avoid chiles that are flaccid, wrinkled, bruised, blemished, or discolored.