This Book Constitutes The Refereed Proceedings Of The 21st International Conference On Parallel And Distributed Computing, Euro-par 2015, Held In Vienna, Austria, In August 2015. The 51 Revised Full Papers Presented Together With 2 Invited Papers Were Carefully Reviewed And Selected From 190 Submissions. The Papers Are Organized In The Following Topical Sections: Support Tools And Environments; Performance Modeling, Prediction And Evaluation; Scheduling And Load Balancing; Architecture And Compilers; Parallel And Distributed Data Management; Grid, Cluster And Cloud Computing; Distributed Systems And Algorithms; Parallel And Distributed Programming, Interfaces And Languages; Multi- And Many-core Programming; Theory And Algorithms For Parallel Computation; Numerical Methods And Applications; And Accelerator Computing.