A BLT is a type of sandwich, named for the initials of its primary ingredients, bacon, lettuce, and tomato. It can be made with varying recipes according to personal preference. Simple variants include using different types of lettuce or tomatoes, toasting or not, or adding mayonnaise. More pronounced variants include using turkey bacon or tofu in place of regular bacon, removing the lettuce entirely, or adding other ingredients such as a fried egg, avocado (BLAT), or sprouts.

Although its ingredients have existed for many years, there is little evidence of BLT sandwich recipes before 1900. In the 1903 Good Housekeeping Everyday Cook Book, a recipe for a club sandwich included bacon, lettuce, tomato, mayonnaise and a slice of turkey sandwiched between two slices of bread. While the 1928 book Seven Hundred Sandwiches by Florence A. Cowles includes a section on bacon sandwiches, the recipes often include pickles and none contain tomato.

The BLT became popular after World War II because of the rapid expansion of supermarkets, which made its ingredients available year-round. The initials, representing "bacon, lettuce, tomato", likely began in the U.S. restaurant industry as shorthand for the sandwich, but it is unclear when this transferred to the public consciousness. For example, a 1951 edition of the Saturday Evening Post makes reference to the sandwich, although it does not use its initials, describing a scene in which: "On the tray, invariably, are a bowl of soup, a toasted sandwich of bacon, lettuce and tomato, and a chocolate milk shake."

A 1954 issue of Modern Hospital contains a meal suggestion that includes: "Bean Soup, Toasted Bacon Lettuce and Tomato Sandwich, Pickles, Jellied Banana Salad, Cream Dressing, and Pound Cake." By 1958, Hellmann's Mayonnaise advertised their product as "traditional on bacon, lettuce, and tomato sandwiches", suggesting that the combination had been around for some time. However, there are several references to a "B.L.T." in the early 1970s, including in one review of Bruce Jay Friedman's play entitled Steambath titled: "A B.L.T. for God – hold the mayo." The abbreviation used in title references a line of dialogue in the play in which God yells, "Send up a bacon and lettuce and tomato sandwich, hold the mayo. You burn the toast, I'll smite you down with my terrible swift sword."

According to a 2019 poll by YouGov, it is the fifth-most popular sandwich in the United States, tying with the ham sandwich and falling behind the roast beef sandwich, turkey sandwich, grilled chicken sandwich, and grilled cheese sandwich. A 2008 poll by OnePoll showed that the BLT was the "nation's favourite" sandwich in the UK. BLT sandwiches are popular especially in the summer, following the tomato harvest.

While there are variations on the BLT, the essential ingredients are bacon, lettuce, and tomato, between two slices of bread (usually white), often toasted. The quantity and quality of the ingredients are matters of personal preference. The bacon can be well cooked or tender, but as it "carries" the other flavours, chefs recommend using higher quality meat; in particular, chef Edward Lee states "Your general supermarket bacon is not going to cut the mustard."

Iceberg lettuce is a common choice because it does not add too much flavour while adding crunch. Food writer Ed Levine has suggested that BLT does not require lettuce at all, as it is "superfluous", a suggestion that Jon Bonné, lifestyle editor at MSNBC, described as "shocking". Michele Anna Jordan, author of The BLT Cookbook, believes the tomato is the key ingredient and recommends the use of the beefsteak tomato as it has more flesh and fewer seeds. Similarly, chef and food writer J. Kenji Lopez-Alt believes that a BLT is not a bacon sandwich with additional ingredients, but rather, a tomato sandwich seasoned with bacon. For that reason, Lopez-Alt argues that the BLT is a seasonal sandwich best made with high-quality summer tomatoes.

The sandwich is sometimes served with dressings, like mayonnaise. The bread can be of any variety, white or wholemeal, toasted or not, depending on personal preference.