This chapter explores the existing literature on omnichannel retailing and provides the theoretical grounding for understanding omnichannel customer behaviour. The chapter begins by synthesizing the definitions of omnichannel retailing and omnichannel marketing, followed by the authors’ own classification of seven major research themes in omnichannel-related literature. As the omnichannel revolution continues to reshape consumer expectations and behaviours across customer journey, studying omnichannel customer behaviour has become increasingly important. Building on foundational work in omnichannel customer behaviour, the authors then trace the origins of switching behaviours, their connection to channel choice and customer journey research, and their impact on customer experience. The chapter categorizes various patterns of switching observed in current research and synthesizes seminal studies that have shaped the field. From this review, three critical research gaps are identified: reasons why omnichannel customers switch channels, the factors driving their channel choices during switching, and the lack of segmentation frameworks for omnichannel switchers. This chapter explains more about the omnichannel revolution, its development, and emerging research directions that define omnichannel customer behaviour today.