Lime is typically served with Pad Thai, most often as a wedge placed alongside the noodles. Diners squeeze it over the dish to add fresh acidity and brighten the sauce.
That lime wedge is usually a finishing condiment, so its flavor can be adjusted at the table. Pad Thai’s main sauce may already contain souring ingredients such as tamarind pulp or juice, but fresh lime provides a sharper, more immediate citrus note. It also cuts through the richness of oil, egg, tofu, shrimp, or other proteins.
Pad Thai is known for balancing several tastes at once: sweet sugar, salty fish sauce or soy sauce, sour tamarind or lime, and savory ingredients. Bean sprouts, Chinese chives, crushed peanuts, and chili are also common accompaniments, giving the dish contrasting textures and flavors.
Lemon can appear as a substitute outside Thailand, especially where fresh limes are harder to obtain, but it is not the traditional answer to this question. Grapefruit and orange are not standard Pad Thai garnishes. The lime is generally served fresh and separately rather than cooked until its aroma disappears.