Transepithelial/transendothelial electrical resistance (TEER) is a widely accepted electrical parameter to assess barrier integrity and the suitability of in vitro cellular barriers for transport studies. TEER measurement has advantages over traditional permeability measurements as a quick, label-free, and noninvasive method. An added benefit of TEER measurement is that it can be performed in real-time if the measurement electrodes are integrated into a microfluidic organ-on-chip device such as blood-brain barrier (BBB)-on-chip. As evidenced by the literature, TEER measurements for various cell types can be performed with both commercially available equipment and custom-built microfluidic implementations. The design versatility offered by BBBs-on-chips, coupled with the multitude of variables influencing TEER measurements, can introduce complexity when attempting to compare TEER results across various research laboratories. To establish meaningful comparisons and consensus among TEER measurements reported from different BBBs-on-chips, a comprehensive grasp of the diverse influencing factors is essential. The aim of this chapter is to introduce TEER and its significance, explore the different TEER measurement protocols along with their strengths and weaknesses, and review numerous factors that affect TEER.