**1. Introduction**

The "*phosphorescence oxygen analyzer*" and its use to monitor O2 consumption by cells and tissues are discussed in this chapter (Lo et al., 1996; Souid et al., 2003). This analytical tool assesses bioenergetics in cells undergoing apoptosis (e.g., the mitochondrial cell death pathway), in cells exposed to toxins (e.g., loss of viability) and in cells with a genetically altered energy metabolism (e.g., mitochondrial disorders) (Tacka et al., 2004a-b; Tao et al., 2007; Tao et al., 2008a). This method is applicable to suspended (e.g., Jurkat and HL-60 cells) and adherent (TU183 human oral cancer cells) cells and to fresh tissues from humans (e.g., lymphocytes, spermatozoa and tumors) and animals (e.g., liver, spleen, heart, pancreas and kidney) (Badawy et al., 2009a-b; Whyte et al., 2010; Al Shamsi et al., 2010; Al-Salam et al., 2011; Al Samri et al., 2011). The analyzer allows investigating anticancer compounds (single agents or combinations) for dosing, order of administration and exposure (Jones et al., 2009; Tao et al., 2008b; Souid et al., 2006; Goodisman et al., 2006; Tao et al., 2006a-b; Tack et al. 2004b). It can also be used to monitor reactions consuming or producing O2 (Tao et al., 2008b; Tao et al., 2009).
