β-lactones contain a strained four membered heterocyclic ring. Consequently, they differ from the gamma and delta-lactones both in the general methods available for their preparation and their chemical reactivity. Because of their high order of reactivity, only those β-lactones with favorable physical properties can be isolated readily in pure form. The highly reactive β-lactones are customarily prepared in order to transform them into other products. This chapter will consider, first the preparation of β-lactones, and second, their reactions of synthesis value.