Publisher Summary This chapter discusses the recent developments in the study of Lambda (λ) phage gene expression showing that post-transcriptional control of phage genes by host functions provides highly sensitive regulatory circuits. The control of transcription initiation from the early promoters of bacteriophage λ is not sufficient to provide for subtle controls involved with the switch between lysogenic and lytic pathways. The ability of λ to choose between lytic and lysogenic development has evolved as a response to changes in the physiological state of the host cell. The frequency by which the phage enters the lytic or lysogenic pathway is determined in large measure by the nutritional state of the host cell. For example, starved cells lysogenize more efficiently than cells grown in a rich medium. In addition, the number of infecting phage particles per cell is also known to determine the rate of lysogenization: the higher the multiplicity of infection, the higher the rate of lysogenization. It is probable that the physiological state of the cell is signaled to the phage by host regulatory factors.