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, when they first appeared in advertising and other publications.
The natural insulative properties of sheepskin 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).

Since humans first cultivated sheep, cultures have made soft sheepskin boots. Their popularity diminished once leather tanning of more durable leather products like cowhide increased. Unlike cowhide, the sheepskin is soft and easily abraded, limiting it to gentle use. Sheepskin boots gained in popularity as a result of World War I and World War II, when they were popular with aviators because of the need to maintain warmth in non-pressurized planes at high altitudes. An exhibit of a WWI aviator's outfit in the Canadian War Museum cites the term "fug" boots. Ugg Boots have also been popular with surfers and competitive swimmers since at least the 1960s, for keeping warm while out of the water.[citation needed] Uggs became ubiquitous among Southern California surfers and Southern California downhill skiers, and from there, uggs were popularized by Hollywood into the American mainstream and later the worldwide market.

The combination of its soft shank and sheepskin interior means that ugg boots are designed for casual, short-term use, and not for situations which require sturdy, protective footwear, as the design emphasis is on style and comfort rather than protecting the feet. Cleaning products should never be applied directly to the sheepskin, as this may cause permanent damage.
While in the boot, the sockless foot is in full contact with the sheepskin lining, thereby maximizing the insulative properties of the boot. The mid-calf shaft, while not encasing the entire lower foot in wool, further enhances the thermal qualities of the boot by encasing the ankle and lower portion of the leg. The wide, rounded toebox, wide shaft, and wide heelbox with a heel-less sole enhance comfort by providing a non-constricting space for the foot. The durable sole in the classic ugg measures at the toe about ½ inch, at the arch ¾ inch, and at the heel 1 inch. The thick padding of the sole increases foot comfort during movement, while the soft shank minimizes the weight of the boot.