Ugg boots (sometimes called uggs or ug boots) are a style of unisex sheepskin boots lined with wool on the inside and with a tanned outer surface. Ugg boots often have a synthetic sole, although this is not universal. Uggs have been considered a fashion trend since the early 2000s.

 In the Arctic and China, sheepskin boots have long been popular with people in rural occupations (such as sheep shearers, who have ready access to the raw materials needed to make them). They are traditionally used for skiing. Their popularity increased as a result of World War I and World War II, during which they became popular with aviators because of their need to keep warm in unpressurized planes at high altitudes. Ugg boots have also been popular with surfers and competitive swimmers since at least the 1960s, for keeping warm while out of the water.

The terms ugg boots, ugh boots and ug boots have been used as generic terms for sheepskin boots in Australia since the 1970s,[1] when they first appeared in advertising and other publications.

The natural isolative properties of sheepskin that gives thermostatic properties to the boots: the thick fleecy fibres on the inner part of the boots allow air to circulate and keep the feet at body temperature. This means that Ugg boots can be worn without socks (even in relatively cold weather).