The partial aim of the thesis was to summarize the issue of hemiparasitic plants occurring in Europe. We defined hemiparasites, their functioning, impact on certain hosts under particular conditions and methods of protecting woody plants from hemiparasites. We described the impact of hemiparasites on the physiology of threatened plant species. We compared yellow mistletoe and European mistletoe with similar studies concerned with their characteristics, their impact on hosts etc. We also evaluated results of dataset measured on oak and yellow mistletoe in the locality of Duchonka in august 2019. We characterized the research area, described research design and equipment used for measurements. In order to determine stress, we compared water potential, photosynthesis, transpiration, and mineral nutrients values which include macro elements such as carbon and nitrogen. We found that yellow mistletoe has lower water potential than infected and non-infected oaks. The reason is that yellow mistletoe increases natural drought in habitat by having high stomatal conductance. Yellow mistletoe has a higher transpiration rate than healthy or infested oak because yellow mistletoe does not suffer from water deficiency, whereas it draws water from its hosts with the help of haustoria which are attached to tracheas of oaks. Healthy oak has a higher assimilation rate than yellow mistletoe and infected oak. It has enough water, nutrients, assimilates, and energy needed for photosynthesis, as it is not infested by hemiparasite. Infested oak and yellow mistletoe have lower assimilation rate due to lack of energy and minerals deficiency. Paradoxically, most carbon was found in the leaves of damaged oak. On the other hand, yellow mistletoe has low carbon values because it has a lower effectiveness of photosynthesis which obviously cannot be fully compensated by intake of substances from the host. It also has nitrogen deficiency and a reduced rate of photosynthesis, for which it needs enough minerals, energy, assimilates and water. The work points to a significant impact of hemiparasites on the physiological processes of host woody plants. Hemiparasites damage wood, limit physiological processes, weaken infested trees, which become sensitive to climate conditions, abiotic and biotic factors, or other parasites.