During the last 10 years new evidence has come to light which shows that the biology of neurotransmitters has expanded beyond their traditional role as chemical messengers, which is the release from a neuron, diffusion across a synaptic cleft, binding to and stimulation of a post-synaptic cell. These external signaling substances of the nervous system have been found to exert a strong influence on cells of the immune system and tumor cells. The latter express neurotransmitter receptors and several studies demonstrate the involvement of neurotransmitters in tumor cell progression and metastasis development. Besides their impact on the migration of lymphocytes, which is of primary importance for an anti-tumor response, neurotransmitters comprise a multitude of other immunomodulatory properties, which differ depending on the cell type and cell function. To illuminate the interplay between the nervous system, the immune system and tumor cells, we herein summarize in vitro and in vivo experiments on the effects of neurotransmitters on the migratory activity, proliferation and survival of tumor cells, as well as on the function of leukocytes.