Processing of materials in a high magnetic field has received much attention over the past decades. Phase transformations have a decisive effect on structures and properties of materials. Many studies have indicated that a magnetic field can change thermodynamics and kinetics of phase transformations and further modify microstructures, morphologies, compositions and performance. The article reviews development of phase transformations in a high magnetic field. Methods used to detect phase transformations are briefly described. The effect of a magnetic field on thermodynamics and kinetics of phase transformations in ferrous magnetic alloys is described. In addition, the thermal treatment and grain boundary engineering in a magnetic field are discussed. Studies on phase transformations and structural morphologies of non-magnetic materials in a magnetic field are surveyed.