This book constitutes the refereed proceedings of the Third International Symposium on Practical Aspects of Declarative Programming, PADL 2001, held in Las Vegas, Nevada, USA in March 2001. The 23 revised full papers presented were carefully reviewed and selected from a total of 40 submissions. Among the topics covered are Mu-calculus, specification languages, Java, Internet programming, VRML, security protocols, database security, authentication protocols, Prolog programming, implementation, constraint programming, visual tracking, and model checking.