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.