This paper studies the archival problem: how a digital library can preserve electronic documents over long periods of time. We analyze how an archival repository can fail and we present different strategies that help solve the problem. We introduce ArchSim, a simulation tool that for evaluating an implementationof an archival repositorysystem and compare options such as different disk reliabilities, error detection and correction algorithms, preventive maintenance, etc. We use ArchSim to analyze a case study of an Archival Repository for Computer Science Technical Reports.