The basic features of diphtheria, especially clinical aspects, differential diagnosis and therapy are set out insofar as they are important for the general practitioner. The doctor, not the bacteriologist, must make the diagnosis. Serotherapy must be begun as rapidly as possible, and in adequate dosage. The patients must be carefully watched over in the interests of early detection of possible complications. Rest in bed is required in every case for at least 3 to 4 weeks, longer if possible. Even mild "localized" forms can lead to toxic damage during the course or subsequently. In infants, diphtheria may run its course as the well-defined clinical picture of "diphtheria intoxication" of infants.