Brain training and whether or not it works has been the focus of many controversial discussions in recent years. There are numerous commercial products claiming to improve general mental ability; however, the scientific evidence for such claims is sparse. In order for brain training to be effective, we want trainees to not just get better at the training task, but we also want them to be are able to ‘transfer’ their skills to other tasks or domains. For over a decade, I and my collaborators have been using brain training as a tool to investigate brain plasticity, and we have been developing interventions targeting participants across the lifespan. Our work suggests that cognitive training can indeed lead to generalizing effects in various relevant domains, nonetheless, our data also reveal important factors that constrain the efficacy of training. In order to make our interventions more effective, our more recent work has been focusing on several factors, such as the distribution of training sessions (i.e. spacing), gamification, or the combination of cognitive training with transcranial direct current stimulation. I will conclude by outlining some of the current outstanding questions in this field and how to address them.