Customer engagement has been the holy grail of marketers, especially digital marketers, for the past few. With a better understanding of their engagement, marketers can increase the ROI of their marketing actions and ultimately the customer experience. The Key Engagement Score is the first holistic and audience-centric metric of engagement. It was developed as part of a 4-year academic project surveying hundreds of global consumers and over 50 brands online (e.g. Apple, Starbucks, Disney, Nutella, Coldplay, Louis Vuitton...) The Key Engagement Score is an innovative and holistic method which goes much further than traditional engagement metrics currently proposed online. The Key Engagement Score is based on a holistic approach which integrates affective, behavioral and cognitive metrics and allows capturing the engagement level of any audience regarding a brand, or brand-related object.