This dissertation presents the foundations for building a scalable database management system for managing uncertain data, as it appears in different data management scenarios such as data integration, data cleaning, scientific data and web data management. The result of this work is MayBMS - a scalable open-source database management system for managing large amounts of uncertain data. MayBMS uses the so-called U-relational databases to represent uncertainty. U-relational databases store uncertainty and correlations in a purely relational way, and are a complete representation system for finite world sets. Other benefits achieved by our representation model include compact storage and efficient query evaluation. The results of our experimental evaluation clearly show that query evaluation in MayBMS scales up to large data sizes and uncertainty ratios, and that MayBMS consistently outperforms other current systems for managing uncertain data. The dissertation also discusses optimization of queries on vertically partitioned data, efficient confidence computation algorithms, and challenges and solutions when designing an application programming interface for uncertain databases.