The Chinese style of espionage is not as glamorous as that of the Soviet Union during the Cold War, where large bank deposits, dead drops and videotaping characterized everyday spycraft. It is much more subtle. The Chinese rely on the large number of expatriates in the U.S. for a steady flow of information, which are mainly lay spies. These are students, academics, businessmen and scientists who work at U.S. research institutes and have access to sensitive information. Often these "agents" come to the U.S. for professional reasons and only later find themselves with access to information the government needs. Thus, they either act out of an own initiative or are approached and interviewed by intelligence officers when traveling back to the homeland.
Many cases never come to a conviction for espionage. The reason is that the Chinese government does not usually offer monetary compensation for informants out of fear of leaving a paper trail. Wen Ho Lee, an employee at the Department of Energy's Los Alamos lab, was charged with passing nuclear technology on to the Chinese, but eventually only convicted of tampering with computer files in 1999. Katrina Leung (a Chinese triple agent- yes, this is a real thing - the FBI thought they had broken her allegiance to China, but she secretly fed them disinformation) inflicted serious harm by passing classified information on to the Chinese, but her case was eventually abandoned for "prosecutorial misconduct."
Most of these cases that have surfaced in recent years involved information on U.S. weapons and military technology because, according to the U.S. intelligence community, China is working hard to improve its Air Force, Navy and overall defense technology. Targets are U.S. government as well as private research and development facilities. Collection is facilitated by the often insufficient security measures in private enterprises and contracting companies.
The U.S. counterintelligence community seems aware of the threat, but also needs to become aware that is a new kind of espionage, more clandestine, unorganized and subtle than during the Cold War. Paired with the constant threat that nuclear weapon technology may fall into the wrong hands, not only those of the Chinese, but also those of terrorist groups, this is a grave national security concern.