Modular footwear system

1. A sole assembly comprising a sole unit receiver and the sole unit wherein the sole unit is adapted to be received in a sole unit receiver, the sole unit sized and shaped to approximate at least a forefoot or rearfoot of an intended user, the sole unit having at least one surface portion with a plurality of rows of teeth running generally perpendicular to the longitudinal axis of the sole unit and adapted to engage a set of complementary teeth on a sole unit receiver, and wherein the set of complementary teeth are disposed in a slot in the sole unit receiver.

2. The sole assembly of claim 1 wherein the slot is in a forefoot section of the sole unit receiver.

3. A sole assembly comprising:

a sole unit and a sole unit receiver;

the sole unit having one or more engageable elements, the sole unit and engageable elements being adapted to be received by the sole unit receiver to form a sole unit assembly, the one or more engageable elements each having a corresponding engageable element on the sole unit receiver to form a male-female pair of elements that provides an interference fit that helps secure the sole unit to the sole unit receiver relative to forces that may act generally parallel to the ground facing surface of the sole unit; and

wherein the sole unit comprises at least one portion of a surface with a plurality of adjacent rows of teeth adapted to engage a complementary set of teeth on a sole unit receiver; and

wherein the complementary set of teeth is disposed in a slot in a forefoot section of the sole unit receiver.

4. The sole assembly of claim 3 wherein rows of teeth in the slot of the sole unit receiver are formed in a layer of midsole material.

5. The sole assembly of claim 3 wherein the teeth of the sole unit comprise a thermoplastic material.

6. The sole assembly of claim 3 wherein the sole unit has first and second sets of teeth disposed on opposing surfaces of the portion of the sole unit, each set of teeth are adapted to engage a set of teeth on the sole unit receiver.

7. The sole assembly of claim 3 further comprising a surface that is adapted to be received in a sole unit receiver, the surface including a surface with a texture or pattern adapted to frictionally engage a surface on a sole unit receiver.