We deene a language for representing context-sensitive probabilistic knowledge. A knowledge base consists of a set of universally quantiied probability sentences that include context constraints, which allow inference to be focused on only the relevant portions of the probabilistic knowledge. We provide a declarative semantics for our language. We present a query answering procedure which takes a query Q and a set of evidence E and constructs a Bayesian network to compute P(QjE). The posterior probability is then computed using any of a number of Bayesian network inference algorithms. We use the declarative semantics to prove the query procedure sound and complete. We use concepts from logic programming to justify our approach. Submitted to Theoretical Computer Science special issue on Uncertainty in Databases and Deductive Systems.