What are the most practical and effective security countermeasures that you can take agains espionage?

Foreign intelligence officers or terrorists often rely on the co-operation of a disaffected or vulnerable "insider" to damage your organisation or business. They may also try to place a sympathiser or agent amongst your staff. The measures you take to protect against spying will be influenced by your assessment of the likelihood that you or your organization might be an espionage target.

For many organisations working in non-sensitive sectors, good employment practice will be adequate protection. However, speaking with one of our credit card issuer clients recently, we heard that the threat of organized crime is right at the top with data leakage prevention and insider theft of customer data.