The core functionality for a Z39.50 Client Application is described. This core functionality consists of Connection, Initialization, Search, Present and Disconnection. A Z39.50 Client API is described which provides the core functionality. Also included are brief descriptions of TCP/IP, the abstract syntax ASN.1, BER records and USMARC records. Code for implementing the Client API, TCP/IP access, encoding/decoding BER records and decoding USMARC records are freely available.