A t-shirt (or tee shirt) is a shirt, usually buttonless, collarless, and pocketless, with a round neck and mostly (but not necessarily) short sleeves, that is put on over the head and covers most of a person's torso. The sleeves of the tshirt extend at least slightly over the shoulder but not completely over the elbow (in short-sleeve version).

A shirt that is either longer or shorter than this ceases to be a t shirt. T-shirts are typically made of cotton or polyester fibres (or a mix of the two), knitted together in a jersey stitch that gives a t shirt its distinctive soft texture. Tshirts are often decorated with text and/or pictures, sometimes used to market. Tshirt fashions include styles for men and ladies, and for all ages, including baby, youth and adult sizes.

Tshirts have also become a medium for self-expression and advertising, with any imaginable combination of words, art and even photographs on display.

In the early 50's several companies based in Miami, Florida, began to decorate t shirts with different resort names and characters. The first company was Tropix Togs, under founder Sam Kantor, in Miami.

They were the original licensee for Disney characters including Mickey Mouse and Davy Crockett. Later more companies expanded into the tee shirt printing business including the Sherry Manufacturing Company who were also based in Miami. Sherry started in 1948 by its owner and founder Quinton Sandler as a screen print scarf business but evolved into one of the largest screen printed resort and licensed apparel companies in America.

T shirts typically extends to the waist, although one fashion is for "oversized" t shirts that may extend down to the knees, namely in todays hip hop fashion. A more recent trend in women's clothing involves tight-fitting "cropped" t shirts that are short enough to reveal the stomach. Another popular trend is wearing a "long-sleeved tshirts", then putting on a short sleeved tee shirt of a different colour over the top of the long sleeved shirt. Known as "layering".

According to archaeologists and anthropologists, the earliest clothes were most likely consisting of fur, leather, leaves or grass, and were draped and wrapped or tied about the body for protection from the weather. Knowledge of such clothes remains uncertain, since clothes materials deteriorate quickly compared to stone, bone, shell and metal artifacts. Archeologists did identify very early sewing needles made of bone and ivory from around 30,000 BC, which were found near Kostenki, in the Soviet Union, in 1988.

Ralf Kittler, Manfred Kayser and Mark Stoneking, anthropologists at the Max Planck Institute for Evolutionary Anthropology, have conducted a genetic analysis of human body lice which indicated that they originated approximately 107,000 years ago. Since most humans have very sparse body hair, body lice require clothes to survive, so this suggests a surprisingly recent date for the invention of clothes including t shirts.

Their invention may have coincided with the spread of modern Homo sapiens from the warm climate of Africa, thought to have started between 50,000 and 100,000 years ago. However, a second group of researchers used very similar genetic methods to estimate that body lice originated around 540,000 years ago. For now, the date of the origin of clothes and t shirts remains a mystery.