In times of crisis, communities rise to fill the void of faltering institutions, self-organising to provide essential resources to marginalised populations. From providing relief to survivors of natural disasters, to addressing crises caused by societal failings like poverty, homelessness and unemployment, mutual aid is an important tool for community care and the development of new systems of survival. With mutual aid efforts increasingly entering the digital sphere, some work has investigated how the internet has transformed mutual aid, especially via social media. While such work describes mutual aid across a variety of contexts, we lack a broad understanding of how mutual aid principles translate online and the challenges organisers face in this digital landscape. To address this, we review 19 papers, identifying key characteristics, strategies, and challenges in online mutual aid. In doing so, we aim to enhance our understanding of how technology might foster sustainable community support and solidarity.