Developing vertebrate limb is a model system frequently used to study tissue differentiation and patterning. Research methods based on sophisticated genetic and molecular embryology techni- ques allowed revising some of the longstanding hypotheses in this field of research. One of them proposed the mechanism of limb development along proximal-distal axis based on the existence of Progress Zone. Recent results propose an alternative model of limb differentiation along this axis, so called Predetermination Model.