The challenging concept of preclinical Alzheimer's disease.

Alzheimer's disease (AD) is increasingly recognized as a decades-long process that begins before any first cognitive symptoms. Neuropathology and in vivo biomarker studies have revealed a silent preclinical phase. However, its precise definition and boundaries remain debated. Elucidating the biological events and temporal sequence that characterize this stage is essential, while researchers try to identify the optimal window to prevent or postpone cognitive decline. At the same time, "preclinical AD" raises unresolved challenges in diagnosis, prognosis, therapeutic intervention and ethics. This narrative review synthesizes the current evidence, from clinical characterization to prevention trials and societal considerations, with an emphasis on original findings for amyloid, tau and neurodegeneration biomarkers. We critically contrast the two principal research frameworks that structure the field and examine how each shapes patient classification and trial design. By integrating these lines of evidence, we explore persistent gaps in prognostic modeling and in the clinical efficacy of disease-modifying strategies. The review aims to provide a concise but comprehensive overview of the preclinical concept for clinicians and researchers developing the next generation of preventive interventions for AD.