Affordable and easy-to-administer interventions such as cognitive training, cognitively stimulating everyday leisure activities, and non-invasive brain stimulation techniques, are promising avenues to counteract age-related cognitive decline and support people in maintaining cognitive health into late adulthood. However, the same pattern of findings emerges across all three fields of cognitive intervention research: whereas improvements within the intervention context are large and often reliable, generalisation to other cognitive abilities and contexts are severely limited. These findings suggest that while cognitive interventions can enhance the efficiency with which people use their existing cognitive capacity, these interventions are unlikely to expand existing capacity limits. Therefore, future research investigating generalisation of enhanced efficiency constitutes a promising avenue for developing reliably effective cognitive interventions.