Extracellular signal-regulated kinase (ERK) is a key component in the regulation of embryogenesis, cell differentiation, cell proliferation, and cell death. The ERK signaling pathway is altered in various cancer types and is frequently investigated as a target for therapeutic intervention. This application note describes how a live cell FRET assay to study ERK signaling was performed on the Operetta CLS? high-content analysis system. The optimized design of the FRET-based biosensor, the high-quality imaging of the Operetta CLS system and the easy-to-use image analysis tools of the Harmony? software contribute to the robustness of the high-content assay.