'Doulas' (lay women who are trained to support other women during pregnancy, birth and postnatally) can improve outcomes for disadvantaged mothers and babies. This 'realistic evaluation' study uses qualitative interviews to explore the views of staff, commissioners and local champions about the processes of implementing and sustaining five volunteer doula support projects in England. The six key factors in their successful implementation are: meeting local commissioning priorities; staff commitment, expertise and skills; networking with other agencies; defining and marketing the doula role; providing strong support for volunteers; and having some costs absorbed by others. The four key factors in sustaining the projects are: finding ways to balance the numbers of referrals and volunteers; shaping the service to local service drivers; ongoing networking; and responding creatively to funding shortfalls. It is a constant challenge to balance the rate of referrals and the number of trained volunteers within tight budgets and timescales.