This chapter discusses reticular formation (RF), where the term reticular formation is based on the complex network which its constituent neurons and their processes form. The processes establish specific but extensive to and fro connections between the reticular formation and spinal cord, hypothalamus, other limbic structures, cerebellum, thalamus and cerebral cortex. Anatomically, RF may be referred to as medullary, pontine and midbrain RF. The efferent connections of the reticular formation may be divided into ascending and descending projections. The large cholinergic neurons of midbrain and RF project to the cerebral cortex via relays in the thalamus. The small adrenergic neurons spread over the entire length of RF also project extensively to the cerebral cortex via relays in the intralaminar nuclei of the thalamus. RF influences all autonomic functions through its connections with the limbic system and hypothalamus. Reticular formation forms the core of the brainstem.