Protein is built from 20 amino acids, nine of which your body can't produce on its own — these are the "essential" amino acids, and you have to get them from food. A "complete" protein contains all nine in meaningful amounts; an "incomplete" protein is missing or critically low in at least one.
Which foods fall where
Nearly all animal proteins — meat, poultry, fish, eggs, dairy — are complete. Among plant foods, soy, quinoa, buckwheat, and hemp are the notable complete exceptions. Most other plant proteins are incomplete in a predictable, well-documented pattern: grains (rice, wheat, oats) run low in lysine; legumes (beans, lentils, chickpeas) run low in methionine. Because their weak points are different, combining a grain with a legume in the same day — rice and beans, hummus and pita, lentils and toast — produces a complete amino acid profile even though neither food alone is complete.
The myth this created
For decades, popular nutrition advice (traceable back to Frances Moore Lappé's 1971 book Diet for a Small Planet) insisted these complementary proteins had to be eaten in the same meal to "count." That specific claim has been walked back by later research, including Lappé's own later editions: your body maintains a rotating pool of amino acids from recent meals, not just the last one, so eating beans at lunch and rice at dinner still lets your body assemble complete protein over the course of the day.
Does it actually matter for you?
If you eat any animal protein at all — even occasionally — completeness is a non-issue; you're covered. If you're fully vegan, it still isn't something to obsess over meal-by-meal, but it is worth being loosely aware of over the span of a day or two, especially if your diet leans heavily on a single plant protein source. See our plant protein pairing guide for the practical combinations.