A warming device for use in cold environments has a flexible pocket with at least one triangular side gusset for receiving a shoe or boot. The warming device has a receiver for receiving a heat generating packet or fuel stick heating device. The warming device may be firmly attached to a shoe or boot by constricting laces, cords or straps so that the gusset is pleated into loose folds along the side of the boot or shoe with heat generating packet or fuel stick heating device pressed substantially flat against the wearer's instep. The sole of the warming device may be modified to provide an anti-slip surface, for example, by installing studs in the sole. The sole of the warming device may be modified to adapt to specialized shoes or boots, such as ice skates.