Parasitic plants are those that live on or in another plant of a different species and derive their nutrients from their host. This type of association is beneficial to the parasite but harmful or lethal to the host. Parasitic plants are found worldwide, from tropical to sub-arctic regions.

      Parasitic plants are categorized as root parasites or shoot parasites, depending on the portion of the host plant the parasite invades. Root and shoot parasites can be further subdivided into holoparasites, obligate and incapable of independent survival, or hemiparasites, able to survive on their own but with less vigor. Some parasitic plants may have extensive ectophytic (external to the host) systems and/or reduced endophytic (internal to the host) systems, while others may be largely endophytic in nature.

      Root parasites may live underground for most of their life cycle. It is often not known that these parasites are present on the roots until the parasite becomes reproductive and sends floral shoots above ground. A parasitic plant may respond to signals given off by the roots of the host plant which normally attract fungi that are symbiotic with the host's roots. Instead of a helpful symbiont responding, a harmful parasite like Striga or Orobanche is attracted to the signal. The parasite invades by forming a haustorium (usually considered to be a modified root of the parasite) that grows into the host root and taps into its vascular cylinder. The haustorium can then take up water and/or nutrients from the host's vascular system.

      Shoot parasites sometimes have larger ectophytic systems than root parasites. Many times their shoot systems are achlorophyllous and depend on their hosts for needed photosynthates. Cuscuta, the genus of dodder, is an achlorophyllous shoot parasite that has a vining habit. Once the dodder plant makes contact with and invades a host, the portion of the dodder plant rooted in the soil withers and dies. Dodder can easily destroy some crop plants, and if let go to seed, it can be very difficult to eradicate from a cultivated field.

      The mistletoes, (the genera Viscium, Arceuthobium, and Phoradendron) invade tree branches. The mistletoes exhibit varying degrees of parasitism and reduction of the ectophytic system in favor of extensive development of the internal endophytic system. Viscium album has green photosynthetic shoots that are quite extensive and probably are nutritionally capable on their own. Phoradendron species show a more extensive development of the endophytic system. Arceuthobium, the dwarf mistletoe, has no ectophytic system and produces a very widely ramifying endophytic, mycelial-like plant body.