Dr Foyster's field of research is the social history of Britain from the seventeenth to the mid nineteenth century. She specialises in family and gender history. 

She has published on topics such as children and marriage breakdown, remarriage, and parenting.  Her most recent research has been investigating the impact on eighteenth- and early nineteenth-century English families of caring for people with learning difficulties and mental illness.

Her most recent book examines the life of the 3rd earl of Portsmouth, who was the subject of a sensational Commission of Lunacy in 1823.