Recommender systems (RSs), as used by Netflix, YouTube, or Amazon, are one of the most compelling success stories of AI. Enduring research activity in this area has led to a continuous improvement of recommendation techniques over the years, and today's RSs are indeed often capable to make astonishingly good suggestions. With countless papers being published on the topic each year, one might think the recommendation problem is almost solved. In reality, however, the large majority of published works focuses on algorithmic improvements and relies on data-based evaluation procedures which may sometimes tell us little regarding the effects new algorithms will have in practice. This special issue contains a set of papers which address some of the open challenges and frontiers in RSs research: (i) building interactive and conversational solutions, (ii) understanding recommender systems as socio-technical systems with longitudinal dynamics, (iii) avoiding abstraction traps, and (iv) finding better ways of assessing the impact and value of recommender systems without field tests.
RECOMMENDER SYSTEMS -A SUCCESS STORY, MOSTLYPersonalized suggestions for items to buy, news to read or movies to watch are nowadays ubiquitous on the web. Recommender systems are software solutions that generate these suggestions, commonly with the help of statistical models and machine learning techniques. Given their widespread use in practice, their often astonishingly good recommendations, and their proven value for consumers and providers, it is no surprise that research on recommender systems is flourishing. Today, we are witnessing a constantly growing interest in the topic both in academia and industry, with countless papers being published every year.Given this continued research interest, the use of latest deep learning technology also in industry, for example (Steck et al. 2021), and the high quality of many deployedThis is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.