Since the identification and characterization of mistletoe lectins as pharmacologically active constituents at the end of the 1980s, research on mistletoe has made substantial advances. Mistletoe extracts are now available that are standardized in terms of the active mistletoe lectins (measured as mistletoe lectin I, ML I). This constitutes an indispensable precondition for reproducible investigations. Preclinical studies have shown that mistletoe extracts standardized in terms of ML I or pure ML I itself have highly potent cytotoxic and immunostimulating effects, predominantly on the cellular immune system. The immunostimulating effect is correlated with the apoptosis of immunologically active cells at low concentrations. Cytotoxic effects on tumor cells are likewise apoptosis-related, but at higher levels necrotic cell death predominates. Due to these properties, mistletoe extracts or pure ML I showed antitumoral activities in different animal models. The objective of this review is to present the current state of preclinical research on standardized mistletoe extracts which hence may be included in the category of rationalphytotherapy.