Sight reading is a functional skill that is necessary for learning new pieces, chamber music and accompaniment. This study tried a new theoretical approach and was based on a combination of expertise and neuro psychological approaches. A large number of independent variables that fit into the different categories of elementary cognitive skills, general cognitive skills and expertise were used. The division of memory into working and short-term memory, and then the division of short-term memory into music-specific and non-music-specific, was a unique point of this study. Another worthy contribution of this study was the division of sight reading stimuli into five different levels of complexity, which meant that a controlled variation of conditions were implemented in the dependent variable. A crucial point of this study was the use of an objective method of analysing the sight reading performances and having an approach which was cautious and conservative towards data analysis. Another important characteristic of this study was the division of the expertise by age. This paved the way to examine the role of expertise in detail, which had never been done before. From the results of this study, sight reading seems to be a complex combination of skills which consists not only of expertise, but also of cognitive skills and psychomotor optimisation and contributes to the formation of a theory of sight reading with educational consequences. As a further result, it could be shown that the impact of this set of predictors varies with changing task difficulty, and analyses resulted in a dynamic model of predictor variables for each task level.