Body size exerts a critical influence on predator–prey interactions and is therefore crucial for understanding the structure and dynamics of food webs. Currently, predator–prey mass ratio (PPMR) is regarded as the most promising modeling parameter for capturing the complex patterns of feeding links in a simplified way. While PPMR has been widely used in food-web modeling, however, its empirical estimation is difficult, with the methodology remaining controversial. In this talk, I briefly review previous studies of PPMR and point out key methodological problems. The problems arise from the two facts: (i) PPMR can be defined at different biological scales, such as from individuals to communities, and (ii) PPMR may vary with various biological factors, such as species identity and body mass, both of which conflict with the conventional assumptions in food-web models. Further, I introduce recently published data with more appropriate methods and discuss implications for food-web studies. I emphasize the importance of compiling individual-level data on predator-prey body-size relationships for various species and habitats.