The term 'leak' (leaktest, leak test data) over the past couple of years has gained popularity among experts in the field of IT security. It can be seen in the news, and comparative tables, but for many, the scope and purpose of this concepts remain unknown. Why do ordinary computer users should carefully look closely to leak test and their characteristics in their daily computer activity? As the results of tests for leakage of data can help you in choosing a reliable product to protect your PC? This article aims to answer these questions and more. By reading you will get all the information necessary for understanding and correct interpretation of the results of leak tests. What is leak? Leak test – is a tool or set of procedures for identifying the ability of security solutions to resist attempts of illegal shipment of personal information in the network. It shows the readiness of the system block accidental or intentional data leakage. Leak tests originate from the time of the first personal firewalls (firewalls) in the late 90's. The primary task of firewalls that time was to monitor network application behavior and prevent attempts to 'call home' ('to send the stolen data') unlawful applications. Recently, leak tests strongly rooted to modify and the new opportunities such as turning off the firewall protection, the use of new types of cross-program collaboration, the exploitation of vulnerable services – it's all the tricks that hackers use in their 'works' and can be safely attributed a separate category of test programs.