The potassium sodium niobate (KxNa1−x)NbO3 (KNN) family with the perovskite structure is a technologically important lead-free piezoelectric material. This paper reviews the ferroelectric and structural phase transitions of KNN and related materials. The nature of end members, the physical properties, and phase transitions of simple alkali niobate materials MNbO3 (M=Li, Na, K, Rb, and Cs) are reviewed. The binary solid solution's phase diagram, KNN, is introduced concerning the morphotropic phase boundary (MPB). To understand the phase transitions near the MPB composition, the temperature dependences of lattice dynamical properties of KNN single crystals on optical modes and acoustic modes are reviewed by Raman and Brillouin scattering studies, respectively. Physical properties and phase transitions of KNN-based solid solutions were also reviewed.