A key cognitive ability is the selection of information relevant for the control of behavior (attention). Selection is investigated in 'visual search' experiments. Observers indicate the presence of a target object shown, in half of the trials, in an array of distractors. Targets differing from distractors by salient features (a red apple among green apples) are detected rapidly. Targets defined by feature combinations (a red small apple among red large and green large or small apples) are hard to find. Dimension-based models such as 'Guided Search' (Wolfe, 1994) assume that selection occurs in two stages. In stage one, basic features (lines, colors, etc.) are extracted and represented in feature-based modules. In stage two, dimension-based signals are generated coding the relative saliency of display locations. The more features differ from neighboring features, the higher their saliency.Until recently, it was generally accepted that saliency generation is an automatic and stimulus-driven processes and that saliency cannot be modulated by top-down signals. However, research shows that dimension-based saliency signals need to be weighted (amplified) to be able to guide attention and that dimensional weight is top-down modulable. Dimension-based weight patterns persist into the next trials and affect search performance systematically: Repeating the target dimension across trials expedites responses; changing dimensions slows reactions. Thus, dimension-based inter-trial effects constitute markers of dimension-based processing. Objects are characterized by specific features (red, small) rather then dimensions (color, size), thus, it is surprising that inter-trial modulations are dimension-, rather than feature-, based. The present study aims to investigate the functional role and development of dimension-based effects in visual processing in children and young adults. Results of a pilot feature search study with 100 participants aged between six and 20 years suggest that dimension-based effects develop at age eight. Moreover, a decrease in overall reaction times can be attributed to both motor and cognitive development.Four experimental series are proposed to assess the development of dimension-based processing in singleton feature, conjunction, and compound search tasks. Cuing is used to assess top-down effects. Further, the relationship between oculomotor processes and attention is investigated. As little is known about the latencies of saccadic eye movements in children, the research project is expected to contribute both to basic research and to issues in developmental psychology.