This chapter examines the early events underlying antigen-induced B cell activation, with a focus on the critical role of B cell receptors (BCRs). It details the function of tonic BCR signaling for B cell survival and development, highlighting its impact on B cell fate determination. The chapter also explores the initiation of B cell activation by membrane-bound antigens, discussing the conformational change model and the kinetic segregation model. It emphasizes the formation of BCR microclusters, the recruitment of coreceptor CD19, and the subsequent amplification of BCR signaling. Additionally, the chapter investigates the regulation of B cell activation by PKCβ and BTK signaling, as well as the modulation of BCR signaling by PI(4,5)P2 and myristoylated alanine-rich C kinase substrate (MARCKS). The significance of B cell contraction in propagating activation and immunological synapse formation is also discussed, particularly the role of the actomyosin network. This chapter provides a thorough understanding of the molecular mechanisms governing B cell activation, shedding light on the intricate dynamics between BCRs, coreceptors, and signaling pathways.