The article discusses the growing body of evidence that liquid-liquid phase separation (LLPS) underlies the formation of membraneless compartments in cells, such as the nucleolus. It highlights the importance of understanding the biophysical principles and specific properties of biological condensates to gain insights into various biological processes and diseases. The authors propose guidelines for rigorous experimental characterization of LLPS processes in vitro and in cells, emphasizing the need for clear standards due to the expanding range of physiological and pathological contexts involving LLPS. They discuss common experimental approaches and their limitations, and identify experimental and theoretical gaps in the field. The article also provides practical advice on protein production, buffer conditions, and the analysis of physical properties of condensates, including methods for detecting LLPS, characterizing the material states of condensates, and assessing their function in live cells.The article discusses the growing body of evidence that liquid-liquid phase separation (LLPS) underlies the formation of membraneless compartments in cells, such as the nucleolus. It highlights the importance of understanding the biophysical principles and specific properties of biological condensates to gain insights into various biological processes and diseases. The authors propose guidelines for rigorous experimental characterization of LLPS processes in vitro and in cells, emphasizing the need for clear standards due to the expanding range of physiological and pathological contexts involving LLPS. They discuss common experimental approaches and their limitations, and identify experimental and theoretical gaps in the field. The article also provides practical advice on protein production, buffer conditions, and the analysis of physical properties of condensates, including methods for detecting LLPS, characterizing the material states of condensates, and assessing their function in live cells.