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Number magnitude is assumed to be encoded in an analog visuo-spatial frame, which resembles a left-to-right oriented and logarithmically compressed mental number line. Some authors have suggested that also the semantic representation of multidigit numbers can be described by their location on the mental number line. However, more recent behavioural studies suggest that units and decades may be represented complementarily in separate cognitive bins of information. In this dissertation the semantic number representation of one- and two-digit Arabic numbers is examined. The empirical investigations are divided in two parts. The first part concerns activation of semantic number magnitude and its association with spatial processing. In three behavioural studies the spatial nature of the mental number line is examined. In Study 1 the congruity between the mental number line and response location is tested. In Study 2 the congruity between the mental number line and stimulus location is investigated, and in Study 3 the abstractness of number magnitude. The second part of the dissertation is dedicated to cognitive and neurophysiologic correlates of two-digit semantic number processing. In two behavioural Studies 4 and 5 and a combined behavioural/functional magnetic resonance tomography (fMRI) Study 6 the semantic representation of units and decades is explored by means of the unit decade compatibility effect. In Study 4 the influence of different semantic relations on behavioural responses of normal participants is investigated. In Study 5 the influence of reading and segmentation abilities on the semantic representation of units and decades is examined in adult semi-illiterates, and in the fMRI Study 6 the neural correlates of unit and decade representations. Two main conclusions may be drawn from our investigations. First, Semantic number processing seems to be represented in a visuo-spatial code, which shares cognitive resources and interacts with other spatial representations. Second, the mental number line does not provide a comprehensive model for all features of multi-digit semantic number processing. Two-digit semantic number processing activates separate unit and decade representations besides the analog representation of number magnitude.