Al pastor (from Spanish, In the style of the shepherd) is a dish developed in Central Mexico, likely as a result of the adoption of the Shawarma spit-grilled meat brought by Lebanese immigrants to Mexico. Being derived from shawarma, it is also similar to the Turkish doner kebab and the Greek gyros. Whereas shawarma is usually lamb-based (thus the "shepherd style" name), gyros and tacos al pastor in Mexico are made from pork.
Pork is marinated in a combination of dried chiles, spices and pineapple. In some places achiote is also added, and then slowly cooked with a gas flame on a vertical rotisserie called a trompo (lit: spinning top), very similar to how shawarma is cooked, with a piece of fresh onion and a pineapple on top. The juice from the pineapple contains bromelain, an enzyme that breaks down protein and thus makes meat tender. When ready, the meat is then thinly sliced off the spit with a large knife. It is served on small tortillas, with finely chopped onions, cilantro, and occasionally, a small slice of pineapple, and usually topped with some lime juice and hot salsa. This meat is a common ingredient in not just tacos, but also gringas, alambres, huaraches, tortas and pizza.
In some places of Northern Mexico, such as Nuevo Leon, Durango, Chihuahua, these are usually called tacos de trompo if served on maize flour tortillas, and gringas if they are served on wheat flour tortillas with cheese.
A similar dish is called tacos árabes, which originated in Puebla in the 1930s from Arab Mexican cuisine. Tacos árabes use shawarma-style meat carved from a spit, but are served in a pita bread called pan arabe. These tacos have been brought by Mexican immigrants to the United States in the past few years and have become popular in cities like Chicago and Los Angeles, two of the largest Mexican/Mexican-American population centers in the United States.
A non-pork version was brought "back" to the Middle East in the early 2000s, and sold as "shawarma mexici". It is essentially a chicken shawarma made in the Middle Eastern style (wrapped with garlic mayonnaise, dill pickle and french fries in a thin flatbread), with the only difference being the marination of the chicken in the al pastor style.
- David Sterling, "The Lebanese Connection," Yucatan: A Culinary Expedition. 
- David Hammond, "Perfection on a Spit," Chicago Reader, November 8, 2007.