Background: Cognitive training (CT) may be effective as a therapeutic strategy to prevent cognitive decline in older adults. This review evaluates CT as a preventive tool at various stages of a prevention hierarchy with specific reference to healthy older adults, “at risk” and clinical populations. It also considers the underlying mechanism of CT, namely that which suggests that CT acts via promoting neuroplasticity. Methods: Evidence for CT in healthy, “at risk” and clinical populations has been systematically reviewed elsewhere. This review re-examines several studies in each group to clarify the potential of CT as a preventive technique, with a key focus on the secondary level of prevention. Results: Studies in healthy older adults and those with mild cognitive impairment are largely positive and suggest that CT has the potential to improve cognition. However, findings in relation to Alzheimer's disease are mixed. Limitations of existing research include diverse methodologies and CT programs, small samples, insufficient focus on functional outcomes, sustainability and generalization of effects and the need for imaging data to delineate mechanisms of change. Additionally, there is limited data on those with late-life depression, despite this being an independent risk factor for dementia. Conclusions: CT offers promise as a preventive therapeutic technique in healthy older adults and particularly as a secondary prevention method for “at risk” groups. Future investigations need to focus on methodological constraints and delineating possible neuroplastic mechanisms of action. Nonetheless, CT programs may represent a viable, non-pharmacological early intervention strategy, as they are easily-implemented, engaging and promote social interaction in group settings.