Reaction wood possesses altered properties than normal wood and develops in response to gravistimulus in trees. In gymnosperms it is referred as compression wood and develops on the lower side of leaning stems or branches, while in angiosperms it is called tension wood and is formed on the upper side of the leaning stem or branches. Reaction wood performs the function of regulating a tree’s form but at the same time is a serious defect in wood utility. This paper describes the factors which lead to the formation of reaction wood and its different properties.