This book constitutes the refereed proceedings of the 18th International Symposium on Practical Aspects of Declarative Languages, PADL 2016, held in St. Petersburg, FL, USA, in January 2016. The 11 revised papers presented were carefully reviewed and selected from 17 initial submissions for inclusion in the book. PADL is a forum for researchers and practitioners to present original work emphasizing novel applications and implementation techniques for all forms of declarative concepts, including, functional, logic, constraints, etc.