The computational logic PX (Program eXtractor) is used to verify programs, extract programs from constructive proofs, and give foundations to type theories. While it is well known theoretically that programs can be extracted from constructive proofs, this study shows how it can be done in practice. The authors give a precise description of the formal theory of PX, its semantics, the mathematical foundation of program extraction using PX, and several methodologies and their theories of program extraction. They also describe an experimental implementation of PX. Contents: Introduction. Formal System. Realizability. Writing Programs via proofs. PX as a foundation of type theories. Semantics. Implementing PX. Susumu Hayashi is a research associate and Hiroshi Nakano a graduate student, both at the Research Institute of Mathematical Sciences at Kyoto University. PX: A Computational Logic is included in the Foundations of Computing series edited by Michael Garey and Albert Meyer.