As a growing amount of research attention has been diverted, for a number of reasons, from the traditional protein foods to the so-called unconventional food proteins, an awareness has increased of the need to understand the functional properties of these proteins. Some empirical functionality tests have been devised, but it is submitted that many of these could yield misleading information, inasmuch asthey often ignore or even run counter to the environmental interactions to which proteins are exposed in food systems. Some examples are given of the influence of the ionic environment upon one basic functional property of proteins, their solubility in aqueous solution.Proteins are not foods; they are food components or food ingredients. While proteins per se are recognized as being essential dietary nutrients, the common protein foods (meat, fish, eggs and dairy products) owe their widespread appeal to the gastronomic pleasure they afford rather than to their nutritional value. In these foods, the proteins are structural components that contribute specific functional properties directly associated with their popularity as foods.The animal protein foods are expensive both in terms of land requirements and market price. In recognition of the worldwide need for more dietary protein, particularly for low-income groups, there have been extensive efforts to develop low-cost protein foods. Because of the realities of marketing, many of these are frank imitations of the natural animal-derived protein products. Therefore, it becomes important to simulate the functional properties of the animal proteins that make their products so attractive to the consumers. The ultimate objective is to be able to create any type of protein food system from low-cost raw materials. This is quite similar to the situation that existed in the margarine and shortening industries 25 years ago. Their objective then was to attain a maximum of flexibility in the selection of raw materials. That objective was attained primarily through investigations of the fundamental physical and chemical properties of glycerides and their relationship to functional and performance properties. Although the systems, and therefore the problems, are much more complex in protein foods, the big accomplishments once again will derive from relating the chemical and physical properties of proteins to function and performance.There are a multitude of sources of low-cost proteins. These include the major oilseeds, fish and the various types of single-cell proteins. As such, they are only raw materials and the challenge of technology is to convert them into useful food ingredients. Their successful use in foods that can be made and sold at a profit will depend upon their functional properties rather than upon their nutritional qualifications. The proteins from each of these raw materials should be considered to be unique and possessing inherent functional properties not necessarily similar to