Polymorphic2D materials allow structural and electronic phaseengineering, which can be used to realize energy-efficient, cost-effective,and scalable device applications. The phase engineering covers notonly conventional structural and metal-insulator transitionsbut also magnetic states, strongly correlated band structures, andtopological phases in rich 2D materials. The methods used for thelocal phase engineering of 2D materials include various optical, geometrical,and chemical processes as well as traditional thermodynamic approaches.In this Review, we survey the precise manipulation of local phasesand phase patterning of 2D materials, particularly with ideal andversatile phase interfaces for electronic and energy device applications.Polymorphic 2D materials and diverse quantum materials with theirlayered, vertical, and lateral geometries are discussed with an emphasison the role and use of their phase interfaces. Various phase interfaceshave demonstrated superior and unique performance in electronic andenergy devices. The phase patterning leads to novel homo- and heterojunctionstructures of 2D materials with low-dimensional phase boundaries,which highlights their potential for technological breakthroughs infuture electronic, quantum, and energy devices. Accordingly, we encourageresearchers to investigate and exploit phase patterning in emerging2D materials.