The production of reactive oxygen species (ROS) in the body has been shown to play a significant role in the development and progression of numerous diseases. This makes it important to develop a method of detection for hydrogen peroxide (H2O2), the most stable ROS. Several methods such as the use of fluorescent probes and electrochemistry have been utilized in the past to detect the imbalance in ROS levels generated from injured or stimulated cells. An imbalance in the levels of ROS leads to a state of oxidative stress within the body. Different enzymes such as horseradish peroxidase (HRP) and superoxide dismutase have been used in the detection of ROS. HRP is commonly used as the biorecognition element in many H2O2 sensors. Researchers have looked into immobilizing these enzymes onto carbon nanotubes and nanoparticles to increase sensor sensitivity. In this chapter, we present experimental procedures to perform electrochemical quantification of H 2O2, one of the major ROS release from injured cells (macrophages and hepatocytes).