Black tea is a variety of tea that is more oxidized than the oolong, green, and white varieties. All four varieties are made from leaves of Camellia sinensis. Black tea is generally stronger in flavor and contains more caffeine than the less oxidized teas. Two principal varieties of the species are used, the small-leaved Chinese variety plant, also used for green and white teas, and the large-leaved Assamese plant, which was traditionally only used for black tea, although in recent years some green has been produced.

In Chinese and Chinese influenced languages, black tea is known as "crimson tea", a description of the color of the liquid; the term black tea refers to the color of the oxidized leaves. In Chinese, "black tea" is a commonly used classification for post-fermented teas, such as Pu-erh tea; in the Western world, "red tea" more commonly refers to rooibos, a South African tisane.
While green tea usually loses its flavor within a year, black tea retains its flavor for several years. For this reason, it has long been an article of trade, and compressed bricks of black tea even served as a form of de facto currency in Mongolia, Tibet, and Siberia into the 19th century. Although green tea has recently seen a revival due to its purported health benefits, black tea still accounts for over ninety percent of all tea sold in the West.

1. After the harvest, the leaves are first withered by blowing air on them.
2. Then black teas are processed in one of two ways, CTC (Crush, Tear, and Curl) or orthodox. The CTC method is used for lower quality leaves that end up in tea bags and are processed by machines. This method is efficient and effective for producing a better quality product from medium and lower quality leaves. Orthodox processing is done either by machines or by hand. Hand processing is used for high quality teas. While the methods employed in orthodox processing differ by tea type, this style of processing results in the high quality loose tea sought by many connoisseurs.
3. Next, the leaves are oxidized under controlled temperature and humidity. (This process is also called "fermentation", which is a misnomer since no actual fermentation takes place.) The level of oxidation determines the quality of the tea. Since oxidation begins at the rolling stage itself, the time between these stages is also a crucial factor in the quality of the tea.
4. Then the leaves are dried to arrest the oxidation process.
5. Finally, the leaves are sorted into grades according their sizes (whole leaf, broken, fanning and dust), usually with the use of sieves. The tea could be further sub-graded according to other criteria.
6. The tea is then ready for packaging.

Black tea is usually graded on one of four scales of quality. Whole leaf teas are highest quality followed by broken leaves, fanning, and dusts. Whole leaf teas are produced with little or no alteration to the tea leaf. This results in a finished product with a coarser texture than that of bagged teas. Whole leaf teas are widely considered the most valuable, especially if they contain leaf tips.

Broken leaves are commonly sold as medium grade loose teas. Smaller broken varieties may be included in tea bags. Fanning are usually small particles of tea leftover from the production of larger tea varieties, but are occasionally manufactured specifically for use in bagged teas. Dusts are the finest particles of tea leftover from production of the above varieties, and are often used for tea bags with very fast, very harsh brews. Fanning and dust are useful in bagged teas because the greater surface area of the many particles allows for a fast, complete diffusion of the tea into the water. Fanning and dusts usually have a darker color, lack of sweetness, and stronger flavor when brewed.

Generally, 2.25 grams of tea per 180 ml of water, or about a teaspoon of black tea per 6 oz. cup, should be used. Unlike green teas, which turn bitter when brewed at higher temperatures, black tea should be steeped in freshly boiled water. The more delicate black teas, such as Darjeeling, should be steeped for 3 to 4 minutes.

The same holds for broken leaf teas, which have more surface area and need less brewing time than whole leaves. Whole leaf black teas, and black teas that will be served with milk or lemon, should be steeped 4 to 5 minutes. Longer steeping times make the tea bitter (at this point it is referred to as being stewed in the UK). When the tea has brewed long enough to suit the tastes of the drinker, it should be strained while serving.

Plain black tea without sweeteners or additives contains negligible quantities of calories, protein, sodium, and fat. Some flavored tea with different herbs added may have less than 1 gram of carbohydrates. All teas from the camellia tea plant are rich in polyphenols, which are a type of antioxidant.

A 2001 Boston University study has concluded that short and long-term black tea consumption reverses endothelial vasomotor dysfunction in patients with coronary artery disease. This finding may partly explain the association between tea intakes and decreased cardiovascular disease events. In 2006, a German study concluded that the addition of milk prevents vascular protective effects of tea. Theaflavin-3-gallate, a theaflavin derivative found in black tea, could reduce the incorporation of cholesterol into mixed micelle