This collection of essays focuses on three reasoning problems devised by Peter Wason - the selection task, the 2-4-6 task, and the THOG problem - which have had a considerable influence since their invention.; The reasons why people make so many errors in these seemingly simple tasks are still t fully understood. A variety of different theoretical perspectives have been used in trying to explain performance. These include the mental models approach, the pragmatic reasoning approach, and the mental logic approach. This book contains chapters which discuss all these theories. Other chapters review the literature or offer alternative theoretical perspectives. A final chapter by Peter Wason describes how he came to create the tasks discussed.