In Belgium, the “public” side of health coverage is delegated and organized around non-profit private health insurers, namely mutual benefit societies, and as such, it is excluded from the perimeter of EU insurance law and regulation. Over the last two decades, and as a result of various governmental attempts to reduce health expenditure, mutual benefit societies nevertheless developed and managed on their own a variety of complementary coverage. This situation was challenged during the 2000s by for-profit insurance companies seeking to penetrate the market. In this context, they used both Insurance and Solvency II directives in their search for supranational support to challenge the position of mutual benefit societies. In turn, the latter responded by working politically to secure their position at the domestic level. As a result of these political struggles, a reform adopted in 2010 reinforced several features of the Belgian public-private mix by safeguarding the position of mutual benefit societies for complementary coverage. But this same reform also opened the supplementary side of health coverage to competition and aligned it with EU provisions, thus marketizing a share of the public-private mix in Belgium—with recent figures suggesting that this new pillar is now expanding.