This chapter reviews key few methods of genetic biocontrol focusing on those systems that have been developed for crop insect pests. These include "sterile release systems" where e.g. sterilized males produce sperm which carries dominant lethal mutations that result in their offspring failing to develop or prevents functional sperm from being produced. We contrast these systems with "fertile release systems" which involve releasing insects that are capable of producing some offspring but with genetic defects that e.g. mean that fewer females survive. The chapter discusses the varied strengths and weaknesses of these different systems.