The traditional fajita is made with marinated flank steak and mixed up with onions and green peppers, but really, anything can be used. Marinate for tenderness and flavor and what you choose to use is up to you. Try steak, pork tenderloin, shrimp and crab, or chicken breast. The options really are endless.
To marinate the meats start with an acidic marinade that includes vinegar and lime juice. These two will tenderize the meat and the lime juice adds that traditional flavor that makes fajitas so good. Include a little oil, garlic, salt, pepper (cayenne if you want them hotter) and cumin. Of course you can add in your favorites too. Typically the meats used are not too thick so the marinating times will be pretty short. Shrimp or crab meat will only need about 20 minutes, while a flank steak should go about an hour.
Since you will be grilling the ingredients, keep the pieces whole. A full steak, whole chicken breasts, etc. Thread shrimp and crab meat onto skewers and grill them kebab style. As for the veggies, cut them in half or manageable pieces, brush with a little olive oil and grill. When everything is done, slice everything into thin strips and place on a heated platter or a hot skillet.
Serve with plenty of sour cream, guacamole, salsa and warmed tortillas. Another great way to serve fajitas is nacho style. Cut everything into bite-sized pieces, pour onto a bed of chips, top with grated cheese and heat until the cheese melts. Then top with sour cream and guacamole and you've got a great snack or appetizer.