Declarative languages have traditionally been regarded by the mainstream c- puting community as too impractical to be put to practical use. At the same time, traditionalconferencesdevotedto declarativelanguagesdo not haveissues related to practice as their central focus. Thus, there are few forums devoted to discussion of practical aspects and implications of newly discovered results and techniques related to declarative languages. The goal of the First International Workshop on Practical Aspects of Declarative Languages (PADL) is to bring together researchers, practitioners and implementors of declarative languages to discuss practical issues and practical implications of their research results. The workshop was held in San Antonio, Texas, during January 18-19, 1999. This volume contains its proceedings. Fifty three papers were submitted in response to the call for papers. These papers were written by authors belonging to twenty one countries from six c- tinents. Each paper was assigned to at least two referees for reviewing. Twenty four papers were nally selected for presentation at the workshop. Many good papers could not be included due to the limited duration of the workshop. The workshop included invited talks by Mark Hayden of DEC/Compaq Systems – search Center, speaking on /Experiences Building Distributed Systems in ML, " and Mark Wallace of Imperial College Center for Planning And Resource C- trol (IC-PARC), speaking on /ECLi PSe: Declarative Specic ation and Scalable Implementation.