This chapter discusses the processes by which one comes to know the environment, namely sensation, perception, and recognition. Perception builds upon basic sensation by extracting more complex attributes from sensory elements. For example, visual perception includes the ability to detect motion, differentiate colors, and distinguish basic forms. Recognition involves identification of a sensory stimulus via access to and integration of stored representations of previously encountered stimuli. Acquired brain damage and developmental abnormalities may affect each level of processing, including primary sensation, cortically mediated perception, or higher-order aspects of perception or recognition. Negative perceptual symptoms involving hearing may affect primary sensory processes, secondary perceptual abilities, or recognition. Cortical auditory disorder or auditory agnosia refers to a non-specific loss of the ability to discriminate both speech and environmental auditory stimuli. Sensory loss may arise from damage at any point within the somatosensory system.