Accident data play an important role in the identification and evaluation of issues relating to highway safety. In-depth accident data are collected by highly-trained investigators and contain the detail needed to address complex safety issues. Police level data are readily available, but lack the detail needed for addressing complex issues, including those on vehicle-highway compatibi1ity.While in-depth accident data do provide the desired level of detailed information, their use are limited by such factors as high costs associated with data collection, resulting in small sample sizes and lack of representativeness, and difficulty and expenses associated with reconstructing the accidents. This paper provides an overview on how in-depth accident data can be used for assessing issues related to highway safety, citing examples from previous and ongoing studies. The paper also discusses problems encountered in the collection and analysis of in-depth accident data.