The invention provides cationic tetracyclic and pentacyclic bacteriochlorophyll derivatives (Bchls) containing at least one positively charged group and/or at least one basic group that is converted to a positively charged group under physiological conditions, preferably Bchls having an onium group derived from a N-containing aliphatic or heterocyclic radical such as ammonium, guanidinium, imidazolium, pyridinium, and the like or a phosphonium, arsonium, oxonium, sulfonium, selenonium, telluronium, stibonium, or bismuthonium group, or a basic group that is converted to such onium groups under physiological conditions, said groups being bound to one or more of the positions 17 , 13 and 3 of the Bchl molecule by ester or amide bond. The Bchls are useful for photodynamic therapy and diagnosis.