The present volume is a homage to Professor N. J. Mackintosh (1935-2015), an outstanding academic and a dear friend and colleague to many of the participants, as a final tribute after being awarded the Gold Medal by the University of Barcelona (November 2015). Although the topics of the chapters in this book have been freely chosen by the authors (Geoffrey Hall, Anthony Dickinson, John M. Pearce, Ian McLaren, Paula J. Durlach, Irina Baetu to mention a few), as well as the type of contribution (either an empirical paper, a review, or an application), they concentrate on issues that are crucial to the understanding of the basic principles of attention and associative learning (both Pavlovian and instrumental), in humans and also in other animals. In other words, to unravel the nature of conditioning, with a special emphasis on perceptual learning. The final chapter, by Gabriel Ruiz, addresses the importance of the contribution by Professor Mackintosh to the renaissance of animal psychology in Spain, where the Spanish Society for Comparative Psychology (SEPC in Spanish) played a relevant role.