<p>Solidarity is relevant to a wide range of fields of practice and policy. In this Introduction to a Special Issue on solidarity in health and healthcare, we discuss recent developments in the conceptualization of solidarity in healthcare, examine the relationship between solidarity and justice and the tensions between inclusion and exclusion. We argue for the treatment of solidarity as a foundational concept for healthcare, for two reasons: First, because solidarity encapsulates the shared human trait of vulnerability, interdependence, and mortality. Second, because it entails also a possible solution to this shared vulnerability: by enacting solidarity at interpersonal or group levels, or having policies and institutions of mutual support, we can mitigate vulnerabilities and strengthen others - and ourselves. We conclude by arguing that solidarity should even be considered a social determinant of health. This is because the presence or absence of solidarity – in the form of individual and group practices, and in the form of solidaristic institutions – can have tangible consequences for people’s health, ranging from access to healthcare to the effects of social isolation or politically heavily polarized societies.</p>