Grenz rays (ultrasoft X-rays, Bucky rays) have been used in the treatment of benign skin disorders for more than 60 years. The mechanism of action, the clinical effect and the potential carcinogenic effect have remained largely unknown, and many of the reported studies are now obsolete. Recent studies of both the clinical and the basic characteristics of grenz rays have shown that the number of Langerhans' cells decreases in the human epidermis after treatment and that grenz rays can suppress the expression of nickel allergy in sensitive individuals. Also, very good results have been reported in the treatment of psoriasis of the scalp. A large-scale study involving more than 14,000 patients has shown that grenz ray therapy cannot be excluded as a risk factor in the development of non-melanoma skin tumors, but this risk factor is small and can virtually be disregarded when certain therapy recommendations are followed. This article is an attempt to summarize the present knowledge of grenz ray therapy.