Parasitic flowering plants represent a unique ecological adaptation, having evolved away from independent function and toward an increasing dependence on other higher plants for survival. Mistletoe, a common evergreen parasite of woody plants, has played a significant role in human culture for centuries. Throughout history, mistletoe species were nurtured and revered as medicinal herbs and religious symbols. But the role of mistletoe has changed. Its importance in western culture has dwindled to a minor, though enduring, association with the Christmas holiday. In contrast, its significance as a parasite of tree crops and woody ornamentals has increased in recent years. Mistletoe species are studied in efforts to control their pathogenic effects and to gain insight into the evolutionary role played by this family of parasitic flowering plants. The unique characteristics of mistletoe that challenge horticultural researchers have contributed to its enduring role in human life.