Heel lifts or height insoles are a wedge-shaped shoe insert which fits in the heel portion of a shoe, with the purpose of adding elevation under one or both feet for therapeutic purposes. Heel lifts are commonly used as therapy for leg-length differences leading to knee, hip and back pain, to reduce stress on the Achilles' tendon during healing, and for various rehab uses. Heel lifts are distinct from heel pads. The intent of a heel lift is not to absorb shock or spread pressure on the heel, but to raise one foot in order to shift balance and gait. As such, heel lifts should be firm and not compressible, in order to add a constant amount of height without causing the heel to rub vertically in the shoe. Heel lifts can cause various side effects similar to wearing high-heeled shoes, and their use should be prescribed by health care professionals.