Multi-cellular engineered living systems (M-CELS) are purpose-driven living systems with multiple interacting living components. They are engineered for specific goals or functions but take emergence into account during the design process, allowing the final system to emerge through natural and non-natural biological processes. M-CELS research is intended to provide a fundamental engineering understanding that enables a quantitative approach bridging between single cells and organs or organisms. The field is still developing, so M-CELS as a field of research is still rapidly expanding.