The potential of biocontrol agents (BCAs) for plant protection has been known for a long time. Their reduced risk for non-target organisms makes them interesting candidates for use in plant protection. Due to their instability against environmental factors such as UV, humidity and temperature, their application success in the field remains limited. The utilization of living cells is a multi-step approach starting from the strain selection, followed by mass production, formulation, drying and storage and subsequent testing in the field. A well-developed formulation can protect BCAs during the formulation and drying and can improve performance at the place of application, in order to develop living organisms into safe and easily applicable biocontrol agents. This chapter provides an introduction into the key aspects that need to be mentioned when developing biocontrol agents, with emphasis on formulation aspects of living biocontrol agents including viruses for insect pest control.