The prevalence of idiopathic hyperhidrosis is between 1% and 2%. Focal hyperhidrosis has a severe impact on the quality of life and includes limitations in work, social interaction, physical activities, leisure, and emotional and psychological distress. Iontophoresis was first described in the eighteenth century. Its clinical use was manifold, also in dermatology. In 1952 tap water iontophoresis was introduced for the treatment of palmoplantar hyperhidrosis. The most comfortable means of iontophoretic treatment employs pulsed direct current of high frequency (5–10 kHz). Side effects are minimal and transient. Only slight skin irritation or sensations of discomfort may occur during treatment. Electric burns and electric shock must be avoided. The main cause of anhidrosis after tap water iontophoresis is most likely a transient functional disturbance of the secretory mechanism of eccrine glands. It was shown that tap water iontophoresis with pulsed current can extend symptom-free intervals in dyshidrotic palmar eczema. Contraindications for tap water iontophoresis are pregnancy and metallic implants, such as cardiac pacemakers or joint or bone implants. Defects in the skin barrier, which cannot be protected by petrolatum or insulating tape, also represent a transient contraindication.