In order to study functioning of cells it is often helpful to disturb the cellular equilibrium and to analyze the consequences. Ideally this is achieved non-invasively and in a living organism. Light is an ideal medium for this purpose in translucent organisms but can also easily be applied via thin fibres in non-translucent animals. Light-sensitive proteins (photoreceptors) from one organism might render another one light-sensitive if expressed in specific cells. 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. We showed that the Channelrhodopsins from the unicellular green alga C. reinhardtii are Light-gated cation channels which allow fast light-induced depolarization of the plasma membrane (1,2). 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) and other transgene animals. We demonstrated that the Light-activated chloride pump halorhodopsin (HR) from the archaeum Natronomonas pharaonis efficiently hyperpolarizes the plasma membrane and 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. Recently we identified and characterized a further bacterial PAC with high cAMP production.