To combat this immense problem, organisations are working to identify them
One billion 'invisible people' miss out on basics
More than 1.1 billion people worldwide officially do not exist - going about their daily lives without proof of identity.
The issue leaves a significant fraction of the global population deprived of health and education services.
Among these "invisible people" - many of whom live primarily in Africa and Asia - more than one third are children whose births have not been registered, the World Bank's "Identification for Development" (ID4D) programme recently warned.
The problem is particularly acute in geographical areas whose residents face poverty, discrimination, epidemics or armed conflicts.
Vyjayanti Desai, who manages the ID4D programme, says the issue arises from a number of factors, but cites the distance between people and government services in developing areas as major.
Many families are also simply not informed about the importance of birth registration - and the consequences of non-registration, which can include the denial of basic rights and benefits, or an increased likelihood of marrying or entering the labour force underage.
And even if parents are aware of the need to declare a birth, costs can be crippling, says Anne-Sophie Lois, a representative at the United Nations in Geneva and director of the children's aid organisation Plan International.
As a result, millions of children in Africa and Asia first encounter the administration only once they reach school age.
But "birth certificates are often needed to enroll in school" or take national exams, Ms Lois says.
The political climate also discourages many families from allowing themselves to be officially identified.
"People fear to be identified from one ethnic group or from one nationality," says Carolina Trivelli, Peru's former development minister. "The government has sometimes - sadly - preferences for some groups rather than another."
And in many countries, births of children born out of wedlock or as a result of rape are sometimes deliberately concealed for fear of discrimination.
In China, avoiding birth registration was also deliberate for years for fear of repercussions due to the one-child policy.
Beyond being barred from attending school, these children can fall prey to violence ranging from forced labour for boys to early marriage for girls, denounced by Unicef in a 2013 report.
These children can also fall victim to human trafficking.
"The legal invisibility of unregistered children makes it more likely that their disappearance and exploitation will go unnoticed by authorities," Ms Lois says.
To combat this immense problem, organisations are patiently working on the ground to identify these "invisible" people.
Digital technologies have provided a tremendous boost, Ms Lois says, as a way to "increase registration, provide legal documentation of vital events and produce statistics that are complete and accurate".
Ms Trivelli says it also helps that "technology is getting lighter - you can go to the people with very small devices" to gather biometric data on the ground.
Plan International, which launched the campaign Every Child Counts in 2005, has contributed to the registration of more than 40 million children in 32 countries.
The organisation developed a digital strategy: village leaders can download a mobile app capable of notifying the government of births and deaths in their villages.
"Digital birth registration systems not only provide children with a legal identity but also provides governments with a continuous source of information through the collection of data," Ms Lois says.
"This allows them to plan effectively for all services that a child needs, including vaccination programmes and education."