The first Thai dictionary from 1873 CE (2416 in the Thai Buddhist calendar) defines kaeng as a watery dish to be eaten with rice and utilizing shrimp paste, onions or shallots, chillies, and garlic as essential ingredients.

The spiciness of Thai curries depends on the amount and kind of chilli used in the making of the paste.

The word "curry" figures in the Thai language as "kari" (Thai: กะหรี่), and refers to dishes using either an Indian-style curry powder, known as phong kari in Thailand, or to the dish called kaeng kari, an Indian-influenced curry that is made with spices that are common to Indian dishes but less often used in these proportions in Thai cuisine.

Ingredients are traditionally ground together with a mortar and pestle, though increasingly with an electric food processor.

With many curries, the paste is first stir-fried in cooking oil before other components are added in to the dish.

This allows certain flavours in the spices and other ingredients in the paste to develop that cannot be released at the lower temperature of boiling water.

Vegetables and fruit, but also certain tree leaves such as from the Acacia pennata (cha-om) and the Ficus virens (phak lueat), and flowers such as those of the Sesbania grandiflora (dok khae)[12] and banana (hua pli),[13] can be added.

[15] Ingredients were dictated by regional and seasonal availability: both pork and chicken (possibly first domesticated from wild jungle fowl in what is now Thailand)[16] are easily available, and so are many varieties of fish, and shellfish, both fresh water species from the many rivers, lakes and rice paddies, as well as salt water species from the Andaman Sea and the Gulf of Thailand.

Kaffir lime leaves and krachai are often cooked along with the other ingredients but fresh herbs such as Thai basil are nearly always added at the last moment to preserve the full taste and serve as a contrasting note to the flavours of a curry.