This book constitutes the proceedings of the 20th International Symposium on Practical Aspects of Declarative Languages, PADL 2018, held in Los Angeles, CA, USA, in January 2018 and collocated with the 45th ACM SIGPLAN Symposium on Principles of Programming Languages. The 13 regular papers presented in this volume together with the abstracts of 2 invited talks were carefully reviewed and selected from 23 submissions. They deal with functional programming; constraint programming and business rules; prolog and optimization; and answer set programming. .