An article of footwear comprising: an upper having an ankle opening, a pair of side portions (21a, 21b) extending downward from the ankle opening, and a connection element located opposite the ankle opening and extending between the side portions, the connection element being attached to a lower portion of the side portions and forming a bottom portion of the upper, wherein the connection element defines a first aperture extending through a forefoot portion of the connection element and a second aperture extending through the forefoot portion of the connection element; and a sole structure that is securable to the upper and separable from the upper, the sole structure having a foot-supporting element, a first projection connected to the foot-supporting element, and a second projection connected to the foot-supporting element, the sole structure being insertable through the ankle opening such that the foot-supporting element is positioned adjacent an upper surface of the connection element and the first and second projections extend through the first and second apertures, respectively, to provide a ground-contacting surface, wherein at least a portion of the foot supporting element and the first and second projections are formed of a polymer foam material, wherein the connection element forms a first portion of a locking system and the sole structure includes a second portion of the locking system, the first portion being joinable with the second portion to secure the sole structure to the upper, and wherein the first aperture and the first projection are located on a lateral side of the article of footwear and the second aperture and the second projection are located on a medial side of the article of footwear. an upper having an ankle opening, a pair of side portions (21a, 21b) extending downward from the ankle opening, and a connection element located opposite the ankle opening and extending between the side portions, the connection element being attached to a lower portion of the side portions and forming a bottom portion of the upper, wherein the connection element defines a first aperture extending through a forefoot portion of the connection element and a second aperture extending through the forefoot portion of the connection element; and a sole structure that is securable to the upper and separable from the upper, the sole structure having a foot-supporting element, a first projection connected to the foot-supporting element, and a second projection connected to the foot-supporting element, the sole structure being insertable through the ankle opening such that the foot-supporting element is positioned adjacent an upper surface of the connection element and the first and second projections extend through the first and second apertures, respectively, to provide a ground-contacting surface, wherein at least a portion of the foot supporting element and the first and second projections are formed of a polymer foam material, wherein the connection element forms a first portion of a locking system and the sole structure includes a second portion of the locking system, the first portion being joinable with the second portion to secure the sole structure to the upper, and wherein the first aperture and the first projection are located on a lateral side of the article of footwear and the second aperture and the second projection are located on a medial side of the article of footwear.