Mitochondrial oxidative phosphorylation is central for generating ATP and maintaining energy homeostasis in most eukaryotic cells. The ex vivo measurement of mitochondrial oxygen consumption rates in intact cells or isolated organelles is a valuable approach to assess mitochondrial bioenergetics in various experimental conditions. In this chapter, we describe several step-by-step protocols for measuring mitochondrial respiration in intact cells, permeabilized cells (in situ mitochondria), and isolated organelles using both Clark-type polarographic oxygen electrode devices and the newly developed oxygen-sensing fluorophore-based Seahorse technology.