We discuss the possible origin of the duality observed in the quantum Hall current-voltage characteristics. We clarify the difference between "particle-vortex" (complex modular) duality, which acts on the full transport tensor, and "charge-flux" ("real") duality, which acts directly on the filling factor. Comparison with experiment strongly favors the form of duality which descends from the modular symmetry group acting holomorphically on the compexified conductivity. The dissipative IV-characteristics V x (B, I) = R xx (B, I) · I, on the other hand, are extremely non-linear in both phases, degenerating to a linear (Ohmic) relation only when B → B c (Fig. 1, top inset).Shahar et al.[1] discovered that to each B there exists a dual field value B d , such that the dissipative IV-curve V (B, I) (suppressing the now superfluous subscript on V x ) after reflection in the diagonal V = I is virtually identical to the dual IV-curve V d (B d , I d ) in the opposite phase. This is implies thata remarkable relation providing unambiguous evidence of a duality symmetry in the QH system, for a particularly simple quantum phase transition. It reveals a profound1 The Hall bar is a high mobility (µ = 5.5 × 10 5 cm 2 /sV ), low density (n = 6.5 × 10 10 /cm 2 ) GaAs/AlGaAs hetero-structure of size Lx × Ly, aligned with a current I in the x-direction so that R * x = (L * /Ly)ρ * x. The Hall resistance R H = ρ H = ρyx is quantized in the fundamental unit of resistance, h/e 2 ≈ 25.8 kΩ, while the dissipative resistance R D = ρxx/ is rescaled by the aspect ratio = Ly/Lx. connection between the transport mechanisms deep inside the quantum liquid and insulator phases, a symmetry which holds to great accuracy even far from the quantum critical point found at:consistent with the self-dual value of eq. (1). We are only aware of two (related) theoretical frameworks that aspire to account for these data (and which motivated the experimental investigation of duality). The first [4] is based on an effective description of the macroscopic theory that posesses a holomorphic modular symmetry relating the complexified response functionsin different QH phases. It successfully predicts the full phase diagram of the QH system, both integer and fractional, including the position of the quantum critical points governing the scaling behaviour of transitions between QH levels [4]. A "microscopic" 3 interpretation of the symmetry as "particle-vortex duality" was given in ref. [5].The second framework is provided by a "microscopic" model [6] in which a so-called "flux attachment transformation" maps the two-dimensional electron system in a magnetic field onto a bosonic system in a different "effective" field. This "charge-flux duality" resulted in a set of rules, known collectively as "the law of corresponding states", which relates QH states carrying different filling factors ν. It also determines the topology of the phase diagram, but neither the location of quantum critical points 3 Our timid use of the term "microscopic" signals that a rigorous derivat...