This chapter investigates the critical processes of attention and how their maladaptive functioning contributes to various disorders. It begins by outlining different types of attention and presenting psychophysical methods for measuring these cognitive abilities in both human and animal subjects. The discussion then transitions to common attentional deficits, including the use of animal models to study conditions like attention-deficit/hyperactivity disorder (ADHD). A significant portion of this chapter is dedicated to the neuroscience of attention, detailing the roles of key neurotransmitter systems, such as glutamate, dopamine, and acetylcholine, and attention brain circuits, including the default mode network (DMN), the dorsal and ventral attention networks (DAN and VAN), the salience network, and the cortico-basal ganglia-thalamo-cortical (CBGTC) and the cerebello-thalamo-striato-cortical (CTSC) loops. This chapter discusses how attentional processes become disrupted when these neurotransmitter/neural circuits become dysregulated.