Eggplants are very important in many Thai dishes, although you could say their role is more of a "supporting" character. Eggplants are thought to have originated in South-east Asia, and you can still find a seemingly infinite variety of eggplants in Thai markets. Eggplants apparently made their way to India by ancient trade routes, and from there to the Mediterranean.
The most common eggplants on sale in a Chiang Mai market.
The types of eggplants Thais use most are quite different from their large elongated deep purple colored western counterparts. Although small versions of the purple variety can be found, the most commonly used type is called ma-kua pro in Thai. In English it is variously known as brinjal, apple eggplant or just plain Thai eggplant. It is about the size of a ping-pong ball with a curiously variegated skin that zigzags from off-white to dark green. Ma-kua pro is sometimes served raw with chili dips, but it is probably most recognizable as an ingredient of green curries.
In green curries, ma-kua pro is joined by the small pea-sized ma-kua puang. Ma-kua puang are somewhat bitter to taste, although the bitterness is reduced the longer they are cooked. Both types of eggplants can be difficult to find. Frozen green peas simulate the appearance if not the taste of ma-kua puang. The best substitute for ma-kua pro is probably zucchini.
The small purple version of the eggplant is called ma-kua muang lek, which directly translates to "small purple eggplant". These vegetables are sometimes fried with fermented soy beans or an egg yolk batter. Another commonly seen type of eggplant is the yellow eggplant, known in Thai as ma-kua luang. These are about the same size as brinjal, but the skin of the ripe eggplant is bright yellow in color. There's a bit of spiciness to this vegetable, which is why it's sometimes used in spicy Thai salads. Finally, there's a long thin green eggplant that sometimes is known as a Japanese eggplant, or oriental eggplant. In Thai, its just plain ma-kua yao, or "long eggplant."
Eggplants must be cut using a knife with a stainless steel blade. Any other metal will discolor the flesh. Eggplants must be used or served immediately after being cut or else they will turn a rather unappetizing shade of black.