Computer architectures and computer systems are becoming increasingly complex. Multicore processors and heterogeneous computing systems equipped with accelerators pose demanding challenges to programmers when optimizing the performance of their codes. Auto-tuning is a promising technology for tackling these challenges. Several methodologies and auto-tuning approaches have been proposed, especially for numerical computations. More recently, the applicability of auto-tuning has been extended to other areas of computer science and engineering, from embedded systems to extreme scale computing systems.