Hypericin, a naturally occurring pigment, is found in certain species of plants from the genus Hypericum, the most common of which is Saint John's Wort (Hypericum perforatum). Recent interest in hypericin is provoked by the discovery that it possesses extremely high toxicity towards certain viruses notably the class of enveloped viruses that includes human immunodeficiency virus (HIV) and toward tumors, and that this toxicity absolutely requires light. Consequently, a detailed understanding of the interaction of hypericin with cellular components (membranes, proteins, nucleic acids) and with light is of fundamental biological importance. The antiviral and antineoplastic activities of hypericin and its derivatives and its mode of action have been widely studied, in the last two decades. This review is focused on the results obtained in the study of hypericin heteroassociations with biological macromolecules, DNA and human serum albumin in particular. An alternative type of the hypericin photosensitizing activity associated with its ability to produce a photogenerated pH drop is discussed that and discussed in connection with its potential application in photodynamic therapy. In the review, it is also presented how an interdisciplinary approach supported by sophisticated techniques of optical spectroscopy and molecular modeling can be effectively applied for the identification of the specific binding sites of the drug in some biomacromolecules as well as for the determination of the physico-chemical mechanism'of the biological activity of hypericin.