Optogenetics is a fast developing science that combines gene biology and optics to new research fields in neuroscience and cell biology. In the introduction a short literature update of optogenetic tools is given with some emphasis on rhodopsin and flavin based optogenetic tools. Then the BLUF coupled photo activated adenylyl cyclases (PACs) are discussed (BLUF = Blue Light sensor Using FAD). The primary photo-dynamics of the BLUF domains is described. A BLUF domain photocycle scheme and a BLUF domain reaction coordinate scheme are presented. The adenylyl cyclase action of catalytic ATP to cAMP (a cellular second messenger) conversion is described. Light-induced cyclase activity of BLUF coupled photoactivated adenylyl cyclases (BLUF-PACs) is analyzed. Optogenetic BLUF-PAC applications are shortly reviewed. (C) 2016 Elsevier Ltd. All rights reserved.