This paper takes a look at the modeling side of pension fund management. It is based on interviews with pension fund managers, regulators, consultants, and academics in four countries – the Netherlands, Switzerland, the United Kingdom, and the United States. The objective was to understand, through the experience of market participants, the role of modeling in managing defined-benefit pension funds. The 28 defined-benefit pension funds participating in the study have a total of €334 billion ($436 billion) assets under management. The findings of our study show that modeling is now considered an indispensable tool by many market participants. The need to manage the risk inherent in defined-benefit pension plans is the key motivation behind the growing use of modeling. In the Netherlands, for example, where private-sector plans did not experience serious underfunding problems after the 2000 market crash, the use of modeling is widespread and well-integrated in the decision-making process. Dutch regulators have recently mandated a risk-based approach and specified broad principles of sound modeling, including the marking to market of assets and liabilities.