O2 is essential for aerobic organisms.  While this characterization for O2 appears almighty in biology, it is not enough to describe delicate significance of O2 in vivo.  When O2 is highly required to elicit metabolic activity through ATP productio n in  cells and organs, it is very true that their survival is easily endangered by disruption of O2 supply (hypoxia).  Howe ver, recently, evidence has been accumulated that hypoxia can support certain states of cells and organs.  In addition, if s urplus O2 is provided to cells and organs in the body (hyperoxia), it leads to the production of reactive species responsibl e for oxidative stress that influences and often dysregulates downstream cellular signals.  Today, I would like to introduce  some of our works on TRP channels that act as sensors for O2 and reactive oxygen species and also as actuators to initiate downstream electrical/chemical signals, and discuss about how ‘appropriate’ levels of O2 are attained via TRP channels.