The Pension Benefit Guaranty Corporation (PBGC) insures private sector defined benefit (DB) pension plans, when an employer becomes insolvent and is unable to pay the pension liabilities. In principle, the insurance premiums collected by PBGC should be sufficient to cover potential losses; this would ensure that PBGC could pay the insured benefits of terminated pension plan without additional external funding (e.g., from taxpayers). Therefore, the risk exposure of the PBGC from insuring DB pension plans arises from the probability of the employer insolvencies; and the terminating plans’ funding status (the excess of the value of the insured plan liabilities over the plan assets). Here we explore only the second component, namely the impact of plan underfunding for the operation of the PBGC. When a DB plan is fully funded, the PBGC's risk exposure for an ongoing plan is low even if the plan sponsor becomes insolvent. Thus the questions most pertinent to the PBGC are: what key risk factors can produce underfunding in a DB plan, and how can these risk factors be quantified? We discuss the key risk factors that produce DB pension underfunding, namely, investment risk and liability risk. These are interrelated and must be considered simultaneously in order to quantify the risk exposure of a DB pension plan. We propose that an integrated risk management model (an Integrated Asset/Liability Model) can help better understand DB pension plan funding risk. We also examine the Pension Insurance Modeling System developed by the PBGC in terms of its own use of some of the building blocks of an integrated risk management model.