Abstract Emerging science is proposing that plant protein, or some plant proteins, may affect cardiometabolic health for different reasons, starting from amino acid composition and continuing with the other substances and nutrients with which plant protein (as compared to animal protein) is intimately associated. The final picture remains very unclear. Closely controlled trials of supplementation with a specific protein source given as selectively as possible remain little conclusive regarding the true effect of plant proteins on the intermediary endpoints of cardiometabolic risk such as plasma lipid levels. Trials using plant protein foodstuffs rather than purified proteins have yielded more conclusive results, but the effects of protein per se cannot easily be disentangled from those of changes to other nutrients and substances associated with protein. Furthermore, these studies have mostly been restricted to specific protein sources, thus hampering any extrapolation to “plant protein” in general and more generally versus animal protein. Likewise, the effects of interventional trials comparing diets that may differ markedly at many levels cannot be ascribed directly to the pattern of protein sources. Observational studies have been very useful in revealing associations between plant/animal protein intakes using hard outcomes of cardiovascular health and under “real-life” conditions, although the risks of residual confounding factors are important. We have argued that a considerable part of the association between plant and animal protein intakes and cardiometabolic risk could be ascribed to the large cluster of nutrients and various other substances that they directly or indirectly convey, while a lesser part may also be linked to the amino acid content. Whatever the case, an intake of plant protein has consistently been shown to be a robust marker of a healthy diet in developed countries, and it appears to provide a very good basis for recommending the sourcing of more protein from plant foods in developed countries, and maintaining a high proportion of plant protein in developing countries. As this review has emphasized, plant protein is a key nutritional characteristic of a plant-based diet that is healthy provided it is well balanced. Many more studies are now needed to understand how plant proteins per se can affect health.