Summary: In his 1910 textbook, 8th Edition, Mental Illness in Old Age’’, E. Kraepelin stated, “The realm of late-life psychiatric disorders is perhaps the most unclear in the entirety of psychiatry.’’ More than a century later, it remains challenging to fully understand late-life psychiatric disorders, including late-life depression, late-life bipolar disorder, and late-life delusional states such as late paraphrenia. However, recent years have seen significant advancements. Neuropathological examinations of these late-life psychiatric disorders are gradually uncovering the underlying diseases. In addition, progress in neurofunctional imaging studies using positron emission tomography (PET) is shedding light on their neurological foundations. Traditionally, mood disorders and delusional conditions in the elderly were considered distinct from dementia. Yet, over time, more cases are being observed to progress into some form of dementia or neurodegenerative diseases. These cases are suspected to have diverse neuropathological entities based on the type of abnormal proteins accumulating in the brain, such as amyloidopathy, synucleinopathy, or tauopathy. Among these, we have specifically revealed that tauopathy is a background factor in some cases of late-life mood disorders and late-life delusional states, using Florzolotau tau PET imaging. We have also found that psychiatric symptoms like delusions are related to the degree of accumulation of tau proteins. The involvement of tau pathology in symptom formation in late-life psychosis suggests that disease-modifying drugs targeting tau, which may emerge in the near future, could be effective in treating these individuals.