This book constitutes the refereed proceedings of the 12th International Symposium on Practical Aspects of Declarative Languages, PADL 2010, held in Madrid, Spain, in January 2010, colocated with POPL 2010, the Symposium on Principles of Programming Languages. The 22 revised full papers presented together with 2 invited talks were carefully reviewed and selected from 58 submissions. The volume features original work emphasizing novel applications and implementation techniques for all forms of clarative concepts, including functions, relations, logic, and constraints. The papers address all current aspects of declarative programming; they are organized in topical sections on non-monotonic reasoning - answer set programming, types, parallelism and distribution, code quality assurance, domain specific languages, programming aids, constraints, and tabling - agents.