The major purpose of this special issue is to highlight the topic of expert thinking. The issue samples the diversity of domains of expertise and includes a good sample of paradigms and methods, with articles that involve think aloud problem solving tasks, computer simulations, and traditional learning or memory tasks. It also has articles that illustrate the diversity of settings in which expertise is practised and can be studied, ranging from the traditional psychology laboratory to cognition in "the wild". Reasoning is generally regarded as an aggregate of fundamental processes that are involved in such complex behaviours as decision-making, planning, and problem solving. Are complex reasoning processes per se the defining hallmark of expertise? Articles in this special issue particularly highlight ways in which reasoning does depend on memory, e.g., for musical scores (Chaffin & Imreh) and for chess games (Gobet), and does become more efficient over time (Clarke & Lamberts). However, experts also use quite general strategies, such as hypothesis testing and the combination of forward and backward chaining (Clarke & Lamberts, Ball, Evans, Dennis & Ormerod).