Gradients, waves and timers, an overview of limb patterning models.

•Early opposing signalling gradients pattern the stylopod–zeugopod transition.•Proximal RA and distal FGF stand as the likely early limb P-D signals.•Timing mechanisms predominate during later stages to pattern further distal regions.•The mechanisms that establish digit identity and the role of SHH in this process are under debate.•SHH controls the size of the handplate, where the digit number is established by a Turing mechanism.