Common oral diseases, including periodontitis and dental caries, and their endpoint as tooth loss are controllable yet highly prevalent among adults worldwide. Cognitive decline also poses significant global public health challenges during the aging process, especially the pathological form of cognitive decline such as dementia. Dementia is irreversible and is one of the leading causes of death, disability, and dependency in the aging population. Emerging research suggests a bidirectional association between oral diseases and cognitive decline or dementia. This potential link has implications for designing better oral care plans for patients with dementia and recognizing oral diseases as modifiable risk factors for dementia prevention.This chapter provides an overview of the association between oral diseases and cognitive decline, followed by a discussion of current evidence on such associations in two directions: (1) the impact of cognitive decline or dementia on oral health and (2) the role of oral diseases as modifiable risk factors for dementia. We critically evaluate several hypotheses regarding the underlying mechanisms of this association, including (1) life-course hypothesis, (2) shared inflammation and bacterial infection mechanisms, (3) malnourishment mechanism, (4) pain pathway, and (5) sensory feedback pathway.However, the association between oral diseases and cognitive decline or dementia remains controversial due to limited high-quality evidence, particularly from biomedical research. Much of the existing evidence is from observational studies prone to confounding bias, with inconclusive questions about causation and the direction of causality.This chapter concludes by emphasizing the need for future studies with robust methodological designs, including randomized controlled trials, biomedical studies, and innovative research techniques such as Mendelian randomization. Such studies are crucial for disease prevention and enhancing patient care and quality of life. By providing a comprehensive overview, this chapter contributes to an advanced understanding of this field, addresses current study gaps, and suggests future research directions.