This book constitutes the joint refereed proceedings of the 12th International Workshop on Approximation Algorithms for Combinatorial Optimization Problems, APPROX 2009, and the 13th International Workshop on Randomization and Computation, RANDOM 2009, held in Berkeley, CA, USA, in August 2009. The 25 revised full papers of the APPROX 2009 workshop and the 28 revised full papers of the RANDOM 2009 workshop included in this volume, were carefully reviewed and selected from 56 and 58 submissions, respectively. APPROX focuses on algorithmic and complexity issues surrounding the development of efficient approximate solutions to computationally difficult problems. RANDOM is concerned with applications of randomness to computational and combinatorial problems.