This book constitutes the refereed proceedings of the 12th Asian Symposium on Programming Languages and Systems, APLAS 2014, held in Singapore, Singapore in November 2014. The 20 regular papers presented together with the abstracts of 3 invited talks were carefully reviewed and selected from 57 submissions. The papers cover a variety of foundational and practical issues in programming languages and systems - ranging from foundational to practical issues. The papers focus on topics such as semantics, logics, foundational theory; design of languages, type systems and foundational calculi; domain-specific languages; compilers, interpreters, abstract machines; program derivation, synthesis and transformation; program analysis, verification, model-checking; logic, constraint, probabilistic and quantum programming; software security; concurrency and parallelism; as well as tools and environments for programming and implementation.