Family carers of people who have long term illness often experience physical and mental health morbidities, and burden. While there is good evidence to suggest that carers benefit from psychosocial interventions, these have primarily been delivered via face-to-face individual or group-formats. eHealth interventions offer a novel, accessible and self-paced approach to care delivery. Whether these are effective for carers' wellbeing has been little explored. This paper reports the first comprehensive systematic review in this area. A total of 78 studies, describing 62 discrete interventions, were identified. Interventions commonly aimed to promote carers' knowledge, self-efficacy, caregiving appraisal, and reduce global health morbidities. Interventions were offered to carers of people with a wide range of long term illness; dementia has been the most researched area, as reported in 40% of studies. Clinical and methodological heterogeneity in interventions precluded meta-analyses, and so data were analysed narratively. The most popular approach has comprised psychoeducational interventions delivered via an enriched online environment with supplementary modes of communication, such as network support with professionals and peers. Overall, carers appreciate the flexibility and self-paced nature of eHealth interventions, with high rates of satisfaction and acceptability. More studies using robust designs are needed to extend the evidence base.