The Brain–Nose Interface represents a uniquely exposed and anatomically privileged connection between the central nervous system and the nasal cavity. Along olfactory nerves and perivascular pathways traversing the cribriform region, CNS related biomarkers exit the intracranial compartment to the periphery. This clearance architecture links nasal fluid to core processes in neurodegeneration, including early changes in Alzheimer's or Parkinson's disease. Emerging evidence demonstrates that CNS-related proteins can be detected in non-invasively collected nasal fluid. Recognizing the BNI as an easy, safe, and repeatable access to brain pathology will transform strategies for early diagnosis, monitoring, and precision medicine in neurological disease.