Microfluidic devices provide intriguing tools for biology, biotechnology and life science. The small size with volumes in the L to fL scale and the fine control over the microenvironment offer an opportunity to accelerate research and development in these areas, where oxygen is a key parameter. Quantification of oxygen is therefore essential and optical sensors are ideally suited for this task because they can be downsized and easily integrated into microscale devices. The integrated oxygen-sensitive material can be read-out contactless with light through the channel wall. This chapter represents an overview of the integration and application of optical oxygen sensors into microfluidic devices and microreactors. It introduces microfluidics for biological systems and flags the necessity of oxygen monitoring. Challenges of integration and demands on optical sensor materials and detection systems are discussed. Sensor formats, integration methods and detection principles are reviewed. Microfluidic systems with integrated sensors are categorized by their application in microbioreactors, cell culture and tissue engineering and representative examples are showcased.