This book begins with a theoretical and up-to-date overview on automatic and controlled processes. Automatic processing is effortless, fast and fairly error-free. It can be accomplished simultaneously with other cognitive processes without interference, it is not limited by attention capacity and it can be unconscious or involuntary. Controlled processing is effortful, slow and prone to errors but -- at the same time, flexible and useful to deal with new tasks. Some automatic processes are thought to be pre-programmed or innate and include the encoding of temporal or spatial relationships, frequent monitoring and the activation of word meaning. Other cognitive processes become automatic with practice. The second part deals the shift from controlled to automatic processing as the core of the access to complex thinking. When somebody starts learning, attention is allocated in order to fulfil task requirements. Performance requires controlled processing. When training proceeds, performance requires less vigilance, it becomes faster and faster and errors decrease. This is defined automatisation. Automatisation concerns both perceptual and motor skills and cognitive processes. The essence of the book is that high load in the coding of the stimuli results in reduced perception of distractor stimuli because there is insufficient capacity to process them all. The controlled processes rely on and negatively influence higher mental functions, such as working memory, which are required to maintain current priorities and to choose between them, and also rely on complex thinking because this latter ask for an efficient working memory system.