This collection comprises the expanded and fully refereed versions of selected papers presented at the 17th International Workshop on Approximation Algorithms for Combinatorial Optimization Problems (APPROX 2014) and the 18th International Workshop on Randomization and Computation (RANDOM 2014) held at the Universitat Politècnica de Catalunya, Barcelona, Spain, September 4 - 6, 2014. The selection was made by the program committees of the respective meetings (listed below). Preliminary versions of the papers were presented at the workshops and the extended abstracts appeared in the proceedings of the meetings published by Dagstuhl Publishing.

The APPROX Program Committee selected 31 out of 64 submissions for presentation at the Workshop; of these, 2 were invited to this Special Issue. The RANDOM Program Committee selected 30 out of 62 submissions; of these, 3 were invited to this Special Issue. The authors of 4 of the 5 selected papers accepted the invitation.

The papers selected from APPROX 2014 cover topics of expansion in graphs and hypergraphs, and approximation of graph spanners, while the papers selected from RANDOM 2014 cover topics in communication complexity and pseudorandomness.

All papers were refereed in accordance with the usual rigorous standards of Theory of Computing.

We would like to thank the authors for their contributions and the anonymous referees for their hard work that helped improve the quality of this issue. It was a pleasure to edit this special issue for Theory of Computing.

APPROX 2014 Program Committee

Niv Buchbinder (Tel Aviv University)
Deeparnab Chakrabarty (Microsoft Research, India)
Siu On Chan (Microsoft Research, New England)
Shuchi Chawla (University of Wisconsin)
Eden Chlamtac (Ben Gurion University)
Nikhil Devanur (Microsoft Research, Redmond) (Chair)
Alina Ene (Princeton University)
Konstantinos Georgiou (University of Waterloo)
Telikepalli Kavitha (Indian Institute of Science, Bangalore)
Ken-ichi Kawarabayashi (National Institute of Informatics, Tokyo)
Jochen Koenemann (University of Waterloo)
Amit Kumar (Indian Institute of Technology, New Delhi)
Konstantin Makarychev (Microsoft Research, Redmond)
Debmalya Panigrahi (Duke University)
Thomas Rothvoss (Massachusetts Institute of Technology)
Barna Saha (AT&T Shannon Research Laboratory, New Jersey)
Bruce Shepherd (McGill University)
Aravind Srinivasan (University of Maryland)
David Williamson (Cornell University)

RANDOM 2014 Program Committee

Louigi Addario-Berry (McGill University, Canada)
Nayantara Bhatnagar (University of Delaware)
Amin Coja-Oghlan (Goethe University, Germany)
David Galvin (University of Notre Dame, Indiana)
Valentine Kabanets (Simon Fraser University, Canada)
Michael Molloy (University of Toronto)
Cris Moore (Santa Fe Institute, New Mexico) (Chair)
Assaf Naor (New York University)
Krzysztof Onak (IBM T. J. Watson Research Center)
Dana Ron (Tel Aviv University)
Alex Russell (University of Connecticut)
Dominik Scheder (Aarhus University)
Devavrat Shah (Massachusetts Institute of Technology)
Perla Sousi (University of Cambridge)
Mario Szegedy (Rutgers University)
Amnon Ta-Shma (Tel Aviv University)
Thomas Vidick (Newton Institute, Cambridge, UK)