This chapter provides a roadmap to the burgeoning literature on two-sided markets with a specific focus on BtoB market places. On-line intermediation involves two-sided network effects between buyers and sellers, and the implications for optimal BtoB platforms’ tariffs are discussed. The chapter discusses first the monopoly case, drawing attention to the distinction between upfront registration and transaction fees. Then the competitive case is discussed, with different degrees of differentiation, the distinction between single-homing and multi-homing, and different business models. The last section is devoted to non-price issues such as tying, the design of the matching process and the ownership structure.