Time is an essential part of information since it predicates about a constantly evolving real world. In this report, we present what is now considered in the best way to deal with time: mixing transaction time (the time when the information was stored in the database) with valid time (the time when the information is true in the modeled reality). By combining this two times, four types of databases are obtained, which differ in both their capability to support these time concepts and their ability to process temporal information. This report defines a bitemporal database model and presents a survey of selected problems in bitemporal databases. These problems focus mainly on the following issues: representations, updates and query languages for bitemporal databases.