**1.2 Aim of this chapter**

An overview of the development of PC/LC is presented. The optical properties, operation principles, and applications of liquid crystal-based photonic crystal devices are discussed. In Section 1, I introduce the basic knowledge of LC and PC physics to make the reader understand the next section. Section 2 details the operation principles of nonchiral and chiral LC modes to help the understanding of switching mechanisms described in two following sections. In addition, the optical properties of defect modes in different LC states are reported in Section 2. Section 2.1 is "Photonic crystals with a nonchiral nematic liquid crystal," Section 2.2 is "Chiral-tilted homeotropic nematic liquid crystal-based photonic crystal devices," Section 2.3 is "Chiral nematic and cholesteric liquid crystal with photonic crystal devices," and Section 2.4 is "Tristable photonic crystal devices with polymer-stabilized cholesteric textures." The configuration of many types of PC/ LC cell applications, including the design of PC multilayers for applications is schematically depicted in Session 3. To realize the PC applications; Session 3.1 introduces "Electrically switchable liquid crystal-based photonic crystals for a white light laser," Session 3.2 shows "Liquid crystal-based photonic crystals for pulse compression and signal enhancement in fluorescence applications," and Session 3.3 demonstrates "Photo-manipulated photonic devices based on tristable chiraltilted homeotropic nematic liquid crystal." Finally, I will summarize the results and conclude about this chapter.
