Liquid–liquid phase separation (LLPS), has emerged as a new paradigm in the fields of soft matter, colloid chemistry, food science and cell biology. Research in this area constitutes a fine example where physics and biology intertwine harmoniously and not only in the grand Ouest. LLPS is a dynamic assembly process that leads, in solution or in vivo, to the formation of micrometer-sized droplets, which are referred as biomolecular condensates, membrane less organelles, liquid droplets or complex coacervates, depending on the scientific community concerned.1,2 Herein, we focus on LLPS that occurs in binary protein mixtures (heteroprotein systems). We briefly review aspects that are of particular interest: formation dynamics; main driving forces; physical and chemical properties; functions and applications. The challenges and future research directions are discussed.