In this chapter, the basic concept of tissues’ energy metabolism is presented. The involvement of the mitochondria in the process of cellular respiration is described in detail. In addition, the other technologies of various body organ monitoring in vivo are described. We developed and adopted the multiparametric monitoring system that provides critical information related to the physiological and biochemical activities in various organs in the body. In addition, the energy metabolism process taking place in the brain is described in detail in this chapter. In this chapter, the comparison between two types of organs in the body is presented. The brain, as a most vital organ, is compared to three other less vital organs, namely, the liver, kidney, and testis. The simultaneous monitoring of the mentioned four organs is described and discussed.