Libraries are increasingly demanding the ability to exploit their library systems (LIS) using application programming interfaces (APIs), Web services, or other technologies. This issue of ""Library Technology Reports"" explores how vendors of open source library technology are approaching this desire for customization plus an in-depth exploration of trends towards APIs, Web services, and the service-oriented architecture. Topics covered in this issue include: Why Should Libraries Care about Application Programming Interfaces; APIs - Basic Concepts; Vendors and Products - Case Studies and Customer Responses; API Hype and Reality; and, Conclusions and resources.