The complex two-way relationship between the body and brain is crucial for maintaining mental health and cognitive function. Problems with physiological systems can lead to cognitive decline, anxiety, depression, and neurodegenerative disorders. This review examines the neurobiological mechanisms that support this connection, encompassing the nervous, endocrine, and immune systems, as well as the gut-brain axis and the hypothalamic–pituitary–adrenal (HPA) axis. This chapter examines the primary causes of cognitive impairment, including ageing, chronic inflammation, hormonal imbalances, lifestyle factors (such as diet, exercise, and sleep), and chronic stress. It also talks about how these things are related to diseases like Alzheimer's, Parkinson's, and vascular dementia. Mental health disorders are conceptualised as having physiological origins, affected by systemic inflammation, metabolic dysregulation, and pain. In contrast, interventions such as physical activity, mindfulness, pharmacotherapy, and non-pharmacological therapies (e.g., cognitive training and neuromodulation) exhibit potential for alleviating these effects. Future directions emphasise individualised medicine, new technologies (such as AI-driven biomarkers and brain-computer interfaces), and strategies to maintain brain health in the long term. This chapter emphasises the necessity for comprehensive interventions that focus on body-brain synergy to avert cognitive decline and bolster mental resilience by promoting an integrative, multimodal approach.