Research has shown that married men and women have better physical and psychological health and greater longevity than their unmarried counterparts. However, the past 50 years have witnessed changes in the marriage and divorce rates, resulting in more people at older ages who are unmarried or with varied relationship histories. Given the strong association between marriage and health there could potentially be more people at older ages in poorer health, which may be particularly detrimental given the ageing population. Whilst there is much research looking at marriage and physical and psychological health there is little on marriage and physical capability. Physical capability is the capacity to perform the physical tasks of daily living and is predictive of mortality and future social care use. This PhD investigates the relationship between marriage and physical capability at mid to later life using two measures – grip strength and walking speed – from two nationally representative datasets of people aged 50 years and over in England and the USA. Cross-sectional associations between marriage and physical capability are investigated in a comparative analysis between England and the USA, and longitudinal associations through examining changes in walking speed over a ten year period in England. A descriptive analysis of early life circumstances and its association with entry into and exit out of marriage in England and the USA is also carried out. Findings show that married people had both higher levels of current physical capability and a slower decline in physical capability over time than their unmarried counterparts. Much of the “marriage advantage” is explained by their greater wealth, but there were some unexplained associations, particularly among widowed men. There were few gender and country differences in the association. The results of this thesis suggest that marriage is important for maintaining physical capability for people at mid to later life in England and the USA