An important theme of this book is that aging per se does not lead to declining abilities that are needed for safe mobility. Rather, there are a number of medical conditions and other factors that are more likely to occur in the older adult population that increase risk and are, therefore, associated with aging.* In fact, it is not the condition itself that raises the risk of a crash, but rather how the condition influences functional abilities-those abilities that underlie critical driving skills. A fully managed medical condition, such as high blood pressure, may not affect driving at all. This chapter reviews what is known about how specific medical conditions affect crash risk or, in the absence of risk data, functional abilities. We focus here on chronic (enduring, predictable, and stable) rather than acute (short-term, sporadic, and unpredictable) effects (Dobbs, 2005). During acute events (e.g., cardiac infarction or seizure) individuals are not capable of driving safely and should not do so.