Eggs are laid by female animals of many different species, including birds, reptiles, amphibians, a few mammals, and fish, and many of these have been eaten by humans for thousands of years. Bird and reptile eggs consist of a protective eggshell, albumen (egg white), and vitellus (egg yolk), contained within various thin membranes. The most commonly consumed eggs are chicken eggs. Other poultry eggs including those of duck and quail also are eaten. Fish eggs are called roe and caviar.
Egg yolks and whole eggs store significant amounts of protein and choline, and are widely used in cookery. Due to their protein content, the United States Department of Agriculture formerly categorized eggs as Meats within the Food Guide Pyramid (now MyPlate). Despite the nutritional value of eggs, there are some potential health issues arising from cholesterol content, salmonella contamination, and allergy to egg proteins.
Chickens and other egg-laying creatures are kept widely throughout the world and mass production of chicken eggs is a global industry. In 2009, an estimated 62.1 million metric tons of eggs were produced worldwide from a total laying flock of approximately 6.4 billion hens. There are issues of regional variation in demand and expectation, as well as current debates concerning methods of mass production. In 2012, the European Union banned battery husbandry of chickens.