This chapter starts with a short overview of the actual knowledge about functions of boron (B) in all living systems. Thereafter, a more detailed insight into the special topics of B chemistry is given. The behaviour of B in soils, the factors leading to low or phytotoxic concentrations of B in soils and forms of B fertilizer are addressed. The functions of B in plant metabolism are discussed, including some recent information about potential further functions of B in addition to the cross linking of the small cell wall-bound pectin rhamnogalacturonan-II (RGII) and how B deficiency symptoms could be explained. How to assess the B status of plants and soils is explained. The ways to supply B fertilizers in foliar and/or soil applications are stated, considering the differences in B phloem mobility in different species.