This chapter elaborates the attentional processes in spatial stimulus-response (S-R) compatibility. In choice reaction time (RT) tasks, the time needed to respond to a stimulus depends not only on the characteristics of the stimulus and characteristics of the response, but also on the relation between the two. This phenomenon is known as S-R compatibility. Spatial compatibility effects also occur when attributes of the stimulus other than its location define response selection. The color of a stimulus presented on the left or the right side of a display may indicate which hand to use for responding. Two stimuli were presented on the same side of the body midline, and subjects responded by pressing one of two response keys, both of which were located in the same side of space. This experimental condition, it was argued, allows one to distinguish between the attentional and coding hypotheses. The attentional hypothesis predicts that compatibility effects will be absent, because differential hemispatial orienting is eliminated. The spatial coding hypothesis, in contrast, predicts that compatibility effects will be present, because there is a match between the relative spatial position of stimuli and response keys. © 1990, Elsevier Science & Technology. All rights reserved.