The 80/20 rule has been validated by economists, mathematicians and business analysts, yet it remains strangely unnoticed. It cuts across and confounds conventional wisdom and is pregnant with paradox. It undermines most mental models: the rational, Cartesian view of the world. It explains why effort and reward are so poorly correlated. It shows how the universe is wonky. Properly understood and used, it helps people control their world better, and provides the secret of success. Understanding the law and acting on that insight can even make one happier. Exactly 100 years ago, the Italian economist Vilfredo Pareto published his discovery that led to the 80/20 rule - 80 per cent of the money went to just 20 per cent of the earners. Since then, 80/20 has emerged as an amazing but prevalent law of nature - about 80 per cent of the results flow from 20 per cent of the causes. If the important 20 per cent can be isolated, more can be achieved with less effort. 80/20 has been used since 1950 to achieve a tremendous amount. It has been the guiding principle behind the quality revolution that launched the Japanese industrial miracle and has transformed the value of modern consumer goods such as cars and videos. 80/20 has been deployed by the computer hardware and software producers to change work habits and enrich home entertainment. And the use of 80/20 in business strategy has multiplied profits for many firms, by leading them to focus on the few products and customers where they make the most money. 80/20 is a law of the universe, not just of business. It claims dominion over people's lives; it does not clock off when people do. It relates to personal effectiveness, success and happiness. Every single person who has taken 80/20 seriously has emerged with useful, and in some cases life-changing insights. This book makes 80/20 accessible and usable to everyone to make them more effective and centred on the things that make them happy. Richard Koch is the co-author (with Ian Godden) of Managing Without Management.