Photoreceptors from archaea, bacteria, and green algae were molecularly identified in recent years. We could show that some of them are ideal tools to manipulate animal cells by illumination. The Channelrhodopsins from the unicellular green alga C. reinhardtii are Light-gated cation channels which allow fast light-induced depolarization (1,2) of the plasma membrane. Mutations led to a slower photocycle and therefore to Channelrhodopsins with higher light sensitivity. Neuronal expression of Channelrhodopsin-2 (ChR2) yields Light-induced action potentials and Light-manipulated behaviour in C. elegans (3). The Light-activated chloride pump halorhodopsin (HR) from the archaeum Natronomonas pharaonis hyperpolarizes the plasma membrane and therefore allows Light-induced silencing of neurons (4). These two antagonistic rhodopsins may even be expressed in the same cell and still specifically be light-activated with 460 nm for ChR2 and 580 nm for HR. Photoactivated Adenylyl Cyclases (PAC) from Euglena gracilis (5,6) or Beggiatoa spec. (7) are flavoproteins which quickly elevate cytoplasmic cyclic AMP by illumination with blue light in cultured cells and living animals or plants.