Among ion channels involved in the cardiac action potential, hERG channels encode the alpha-subunit that underlies the rapidly activating delayed rectifier K+ current (IKr) in the heart. These proteins play a central role in cardiac action potential repolarization and therefore in the termination of cardiac systole. Their correct function is crucial for normal overall action potential duration and a standard QT interval of the surface electrocardiogram, as alterations in their functional properties or membrane incorporation (by genetic mutations or by secondary drug effects) play major roles in the development of long QT syndrome, a prolongation of the QT interval of the ECG associated with ventricular arrhythmias. The importance of hERG channels resides in their unusual gating, that are slow activation and deactivation, and much faster inactivation/recovery from inactivation. One debated and undetermined issue is whether S4 movement in hERG channels is slow, of if coupling of the gating mechanism to pore opening is. Voltage-clamp fluorimetry has been successfully used to assess S4 movement in other Kv channels. The same technique was thus applied by attaching TMRM to different extracellular areas of hERG channels (S1-S2, S3-S4 and S5P extracellular linkers) and measuring voltage-dependent movements of hERG channels to elucidate slow activation/deactivation. Results show different profiles of fluorescence depending on the location where the dye is attached, but consistently relate to pore opening/G-V in terms of time course and voltage-dependence, whatever the linker studied, leading to the conclusion that a concerted channel rearrangement is responsible for or resultant of slow opening in hERG channels. Such a concerted movement of several parts of the channels might be due to critical interactions that occur during hERG gating, as it is suggested that interactions between: 1) extra-negative charges in S1-S3 and -positive charges in S4, compared to other Kv channels (1), 2) S4-S5 linker and S6 segment (2) and/or 3) between N-terminal and C-terminal regions (3), increase the amount of energy necessary to open/close hERG channels. Interestingly, one residue at top of S4 (E519C) showed a combined fluorescence signal. Adding functional mutations or changing pH allowed to further correlate one part of the E519C fluorescence with pore opening/closing. Another part appears earlier than pore opening, and presents a voltage-dependence that correlates well with gating-charge movement (4). This was confirmed by recording for the first time gating currents of hERG channels from a human cell line. Interestingly, using both techniques, two voltage-dependent components were recorded, one left-shifted to the G-V (likely corresponding to the Q-V), while the right-shifted component’s origin is obscure. The time constant for the fast S4 movement was about 5 ms from both fluorescence and gating current measurements at +50 mV, which is an order of magnitude faster than pore opening (around 60 ms at those potentials), leading to the conclusion that S4 movement is not the rate limiting-step for pore opening in hERG channels. Other important features of our gating current results were the appearance of a slow component after ~20 ms, and a charge immobilization suggested by a time-dependent reduction of the amount of charge return. Our data suggest that re-assessment of mechanisms underlying hERG activation gating is required, particularly with regard to the coupling mechanisms between voltage sensor movement and channel pore opening.