Intelligent algorithms have reached a new level of performance in recent years and are increasingly employed in application areas such as speech and image recognition, data analytics, or recommender systems. The proliferation of these techniques poses a range of new challenges for the design and engineering of interactive systems since they tend to act as black boxes and do not offer the transparency and level of control to the user which is considered a prerequisite for user-centered design in the HCI field. In this position paper, we provide an overview of the broad areas related to intelligent algorithms and HCI that will need further research in the future to make systems useful, usable and trustable.