Heat stress has an effect on all plant processes, for example seed germination, growth, development, reproduction, photosynthesis and respiration. On a molecular level, it causes membrane destabilisation, protein denaturation and the denaturation of other molecules, but also the creation of ROS which causes damage to all parts of the cell and to important macromolecules. Heat-caused destabilisation of thylakoid membranes and the modification of Rubisco activity affects photosynthesis. C4 and CAM plants are more successful in dealing with stress because they have mechanisms that reduce the oxygenation activity of Rubisco. Plant adaptation to heat stress includes two mechanisms: avoidance and tolerance. The avoidance mechanism protects the plant from heat stress and sun radiation, it is for example movement of leaves positions and completion of reproductive cycle in the colder part of the year. The tolerance mechanism includes a much wider approach. The most important strategies are the production of heat shock proteins (HSP) and antioxidants. Some other molecules are osmolytes, hormones and secondary metabolites. Besides HSP, there are other proteins that protect the cell: ubiquitin, dehydrins and LEA proteins. Antioxidants protect from oxidative stress. There are enzyme (catalase, ascorbate peroxidase, superoxide dismutase, peroxide and glutathione dismutase) and nonenzyme antioxidants (ascorbate, glutathione, tochopherol and carotene) and their role is ROS neutralisation. ABA and ethylene have the most important effects among the hormones.