According to recent models on visual attention, both the salience of signals (bottom-up) and the intention to search for particular stimuli (top-down) are determinants for attentional selection. We investigated these mechanisms by varying the top-down set of participants that had to detect either luminance or orientation changes of two symmetrically located bars. Irrelevant changes impaired target detection when they were presented spatially separated to the relevant change. Initial attentional selection was represented in posterior N1 asymmetries and was determined by both the relative salience of orientation changes and a subsequent intentional bias towards relevant stimuli. Only when salient orientation changes interfered with luminance target selection in the N1 time window did an N2pc occur. Thus, the selection of relevant information proceeds in a network whose activation is induced by a dynamic interplay of bottom-up and top-down processes.