Given the right hardware and software, digital information is easy to create, copy, and disseminate; however it is very hard to preserve. At present, it is impossible to guarantee the longevity and legibility of digital information for even one human generation. Migration can be defined as the periodic transfer of digital materials from one hardware/software configuration to another or from one generation of computer technology to a subsequent generation. In 1998, the Council on Library and Information Resources (CLIR) asked the Cornell University Library to undertake a risk assessment of migrating a handful of common file formats. This report is the fruit of their investigation. It is intended to be a practical guide to assessing the risks associated with the migration of various formats and to making sound preservation decisions on the basis of that assessment. The paper starts from the premise that migration is prone to generating errors and provides practical tools to quantify the risks. It organizes migration into a sequence of discrete steps and offer assessment tools to manage each of those steps. The process is presented in a workbook that can guide digital preservation specialists in their day-to-day operations. Two case studies are also presented--one for image files and another for numeric files. Findings and Recommendations are discussed under four areas: Migration Risk Can Be Qualified; Conversion Software; Access to Format Data; and Public Access Archives of Format Information. (Contains 29 references.) (AEF)