Yeast extract agar is a nutrient-rich medium that allows the cultivation of a wide spectrum of bacteria, yeasts and moulds. Yeast Extract Agar is formulated according to the formula described by Windle Taylor for plate enumeration of microorganisms in water. Water can contain large numbers of microorganisms, especially those from soil and vegetation. This medium is also useful for teaching and demonstration purposes using simple organisms. Yeast extract and peptone provide nitrogen compounds, vitamin B complex and other nutrients for growth.

Yeast extract agar allows separate counting of aerobic mesophilic organisms that form visible colonies in this medium after 24 hours incubation at 37°C and those that form colonies after three days incubation at 22°C. The two methods give different results.