The aim of this paper is to investigate the interface between the formal and informal support provided to very old people against a background of increasing need for care and a decreasing number of potential informal caregivers. We used a sample of 323 community-dwelling octogenarians participating in the Swiss Interdisciplinary Longitudinal Study on the Oldest Old (Swilsoo) (n = 1441 interviews). Descriptive analyses and a multilevel model were used to test whether formal and informal services complemented or substituted one another. The study revealed that the amount of informal services increased significantly as the frequency of formal aid increased, indicating that the two networks were complementary in the majority of cases. In 21.2% of the cases, the informal network partly substituded the informal network (as an adjustment) and only in 6.4% of the cases did the informal support end after the formal support had increased (radial substitution). The concern that the introduction of formal services may curb the readiness of relatives and friends to provide care is thus unfounded.