The process of developing a spy to supply classified information from within an adversary’s security or intelligence apparatus—and the act of accomplishing that operation. Penetration is likely to provide information of significant value. Accordingly, a penetration agent is considered the acme of achievement in the counterintelligence field. Inevitably, most intelligence agencies are likely to experience hostile penetration at some level, and as the spies are professionals, not part-time agents, they are less vulnerable to detection.