This chapter discusses bipolar junction transistors. It describes the basic transistor construction with PNP and NPN types. It explains transistor operation with forward biased base-emitter and reverse biased base-collector junctions. It also discusses currents in transistors including minority and majority carriers. Different transistor configurations - common base, common emitter, and common collector - are presented along with their input/output characteristics and operating regions. Key parameters like alpha, beta, and power dissipation are also covered.