The sympathetic and parasympathetic nervous systems are defined anatomically based on the levels of outflow from the spinal cord and brainstem. The sympathetic system originates from the thoracic and upper lumbar spinal segments and is therefore called the thoracolumbar system. The parasympathetic system originates from the brain stem and sacral spinal cord and is called the craniosacral system. Both systems consist of chains of preganglionic and postganglionic neurons, which are synaptically connected in autonomic ganglia. Sympathetic ganglia are situated away from their targets and organized bilaterally in the sympathetic chains and in the prevertebral ganglia. Parasympathetic ganglia are situated close to the target organs. Most autonomic target tissues react under physiological conditions to only one of the autonomic systems. The widely propagated idea of the antagonism between sympathetic and parasympathetic nervous systems is misleading. The adrenal medulla is an endocrine gland made up of cells releasing either adrenaline or noradrenaline. Postganglionic neurons of autonomic pathways contain combinations of neuropeptides colocalized with acetylcholine or noradrenaline. The principal organization of the peripheral autonomic nervous system in submammalian vertebrate groups is highly conserved in evolution over about 500 million years.