The Global Polio Eradication Initiative, established in 1988, has led to the immunization of 2.5 billion children against the disease. In 2014, only 359 cases of poliomyelitis (polio) were reported worldwide, which is a fraction of the estimated 350000 children in 125 countries who were paralyzed annually by the poliovirus before 1988. There are only two remaining countries where polio is still endemic--Afghanistan and Pakistan, while circulating vaccine-derived poliovirus is still causing outbreaks in others, such as Guinea, Madagascar and Ukraine. Spread of the poliovirus from the two remaining endemic countries to Iraq, Israel, the Syrian Arab Republic and other vulnerable countries is a continuing threat. Until polio transmission in endemic countries is interrupted, the whole world remains at risk. Despite recent developments and improvements in polio immunization campaigns in high-risk countries, major challenges remain. The foremost factor responsible for low immunization coverage is poor security, particularly in Afghanistan and Pakistan. Other contributing factors include a lack of population-based immunization registries and electronic records, overestimating vaccination coverage, human error, drop-outs from immunization campaigns, low demand for vaccination in the population and a lack of awareness of, or education about, immunization among parents. Mobile health (mHealth) is defined as medical and public health practices supported by mobile electronic devices. Recently there has been a dramatic increase in mobile phone use and there are now around 7 billion mobile phone subscribers globally, 89% of whom live in developing countries. Thanks to the portability of mobile phones, more people can be reached by phone than through the Internet and they can be contacted quickly and inexpensively wherever they are located. Even a substantial proportion of those living on less than 1 United States dollar a day have access to mobile phones and short message service (SMS) text messages. Mobile phones are thus changing the mode of communication among people globally. Importantly, mobile phone use has also increased in countries with a high risk of polio: good examples are Nigeria and Pakistan, where there were around 170 and 140 million mobile phone subscribers, respectively, in 2014. (1,2) Given increased mobile phone use, mHealth interventions now have great potential to support the polio eradication programme. In fact, they have been used in the recent past. In Pakistan, for example, an SMS-based service was provided in 2010 that enabled parents to send a free text message to report areas missed by the national polio control programme, with the result that a polio immunization team was dispatched shortly after. (3) In addition, text messages about the polio campaign were sent to more than 8 million mobile phone subscribers to raise awareness, especially in high-risk areas. In another study in Pakistan, smart phones incorporating a geographical information system were used to conduct a survey of mobile phone access and use among caregivers of children younger than five years in randomly selected representative clusters in high-risk towns (i.e. administrative areas) within Karachi where polio cases had previously been detected. The data obtained were linked to an automated text message system used to monitor supplementary immunization activities in Karachi. (4) In Nigeria, two pilot studies in 2011 demonstrated that creating maps using a geographical information system and integrating them into the detailed planning of polio vaccination campaigns provided feedback in real time: they gave information about the performance of the vaccination teams and helped identify missed or partially covered settlements. (5) One component of another study in Nigeria included sending SMS text reminders to improve surveillance of acute flaccid paralysis (Oluwasegun Adegoke, personal communication, 2015). …