Complex interventions are criticized for being a “black box”, which makes it difficult to determine why they succeed or fail. Recently, nine proactive primary-care programs aiming to prevent functional decline in older adults showed inconclusive effects. The aim of this study was to systematically unravel, compare, and synthesize the development and evaluation of nine primary-care programs within a controlled trial to further improve the development and evaluation of complex interventions. A systematic overview of all written data on the nine proactive primary-care programs was conducted using a validated item list.