Internet addiction is expected to become one of the plagues of the 21st century and network-holism will soon be recognised as a disease. The Internet is taking more and more time for addicted people, the computer is becoming the centre of their world, while relationships, school, work, other entertainment, and pleasures are no longer important. Internet addiction is often associated with difficulties in establishing interpersonal contacts, with social anxiety, disrupted self-esteem, and the use of ineffective strategies for coping with stress. The virtual world is attractive for users, it gives them: anonymity, escape from reality, the ability to defuse tensions, long-term and free access to entertainment, and the chance for social acceptance. The first research on Internet addiction appeared in 1996. Initially, it was focused on the definition of Internet addiction and patterns of behaviour that differentiate compulsive and functional use of the Internet. Currently, etiological factors leading to addiction and methods of its treatment are being studied. Treatment concepts and basics of behavioural addiction therapy programmes are based on cognitive behavioural therapy methods, motivational interview, and crisis intervention. This article presents the symptoms and consequences of Internet addiction and discusses the Cognitive Behavioural Therapy for Internet Addiction (CBT-IA) developed by Kimberly Young. The efficacy of this therapy has been proven through research.