Apparatus for use with (i) stimulation circuitry that includes a pair of electrodes (26a, 26b), (ii) measuring circuitry, and (iii) a voltage supply, the apparatus comprising a controller comprising: at least one interface configured to couple the controller to the stimulation circuitry and measuring circuitry; and controller circuitry, configured, while the electrodes (26a, 26b) are coupled to skin of a subject, to perform a stimulation procedure by: driving the stimulation circuitry to pass a plurality of stimulating pulses between the pair of electrodes (26a, 26b), driving the stimulation circuitry to pass one or more intraprocedural-reference pulses between the pair of electrodes (26a, 26b), the reference pulses being different from each of the stimulating pulses, receiving, from the measuring circuitry, an intraprocedural impedance between the electrodes (26a, 26b) that is measured using the intraprocedural-reference pulses, and in response to the impedance measured using the intraprocedural-reference pulses, but not in response to any impedance measured using the stimulating pulses, controlling the stimulation procedure: the controller circuitry is configured, while the electrodes (26a, 26b) are coupled to the skin of the subject, to perform a calibration procedure prior to the stimulation procedure, by: driving the stimulation circuitry to pass a calibration-reference pulse via the pair of electrodes (26a, 26b), measuring a baseline impedance between the pair of electrodes (26a, 26b), and characterized in that: setting the voltage supply to a voltage that is slightly higher than the product of the measured baseline impedance and a maximum desired stimulation current, and the controller circuitry is configured to use the voltage supply, set to the slightly higher voltage, for performing the stimulation procedure. at least one interface configured to couple the controller to the stimulation circuitry and measuring circuitry; and controller circuitry, configured, while the electrodes (26a, 26b) are coupled to skin of a subject, to perform a stimulation procedure by: driving the stimulation circuitry to pass a plurality of stimulating pulses between the pair of electrodes (26a, 26b), driving the stimulation circuitry to pass one or more intraprocedural-reference pulses between the pair of electrodes (26a, 26b), the reference pulses being different from each of the stimulating pulses, receiving, from the measuring circuitry, an intraprocedural impedance between the electrodes (26a, 26b) that is measured using the intraprocedural-reference pulses, and in response to the impedance measured using the intraprocedural-reference pulses, but not in response to any impedance measured using the stimulating pulses, controlling the stimulation procedure: driving the stimulation circuitry to pass a plurality of stimulating pulses between the pair of electrodes (26a, 26b), driving the stimulation circuitry to pass one or more intraprocedural-reference pulses between the pair of electrodes (26a, 26b), the reference pulses being different from each of the stimulating pulses, receiving, from the measuring circuitry, an intraprocedural impedance between the electrodes (26a, 26b) that is measured using the intraprocedural-reference pulses, and in response to the impedance measured using the intraprocedural-reference pulses, but not in response to any impedance measured using the stimulating pulses, controlling the stimulation procedure: the controller circuitry is configured, while the electrodes (26a, 26b) are coupled to the skin of the subject, to perform a calibration procedure prior to the stimulation procedure, by: driving the stimulation circuitry to pass a calibration-reference pulse via the pair of electrodes (26a, 26b), measuring a baseline impedance between the pair of electrodes (26a, 26b), and characterized in that: setting the voltage supply to a voltage that is slightly higher than the product of the measured baseline impedance and a maximum desired stimulation current, and the controller circuitry is configured to use the voltage supply, set to the slightly higher voltage, for performing the stimulation procedure. driving the stimulation circuitry to pass a calibration-reference pulse via the pair of electrodes (26a, 26b), measuring a baseline impedance between the pair of electrodes (26a, 26b), and characterized in that: setting the voltage supply to a voltage that is slightly higher than the product of the measured baseline impedance and a maximum desired stimulation current, and the controller circuitry is configured to use the voltage supply, set to the slightly higher voltage, for performing the stimulation procedure. setting the voltage supply to a voltage that is slightly higher than the product of the measured baseline impedance and a maximum desired stimulation current, and the controller circuitry is configured to use the voltage supply, set to the slightly higher voltage, for performing the stimulation procedure.