One in three people worldwide lives in either China, the largest communist country, or India, the largest democracy.
For the moment, China remains the most populous nation, with 1.3 billion inhabitants, followed by India, which is home to 1.1 billion.
But India's higher fertility rate means the gap is narrowing and the UN expects it to overtake China before 2030.
Both countries are also experiencing rapid growth in their urban populations.
In China, the number of people in towns and cities is likely to exceed the number in the countryside by 2015.
India and China have to face the challenge of providing for their ageing populations, just as many Western nations do.
As people live longer and fertility decreases, there will be millions more people in retirement and fewer workers to support them.
It has been suggested that China will have to ease its strict one child policy to overcome the problem.
In India, where only 10% of the workforce is covered by formal pension schemes, there are questions over how the elderly will be supported.
Some experts say such problems could hamper the nations' economic growth.
China's emergence as a world economic power follows years of expansion, with growth of 9% or more the norm.
It is a major exporter and may now be the world's fourth largest economy, having overtaken Italy and possibly the UK and France.
India has also seen dramatic growth - of more than 7% a year - and is the recipient of much foreign investment.
Figures from the Economist Intelligence Unit suggest the US will remain the largest economy in real terms.
But on a measure based on purchasing power, China could overtake the US by 2020.
Economic, social and environmental problems are a concern in India and China.
Vast wealth gaps exist, with the majority of people left on the margins of the nations' rapid economic growth. Social discontent has affected both.
Air and water quality is a concern in both countries. Many of the world's most polluted cities are in China.
Despite such problems, however, it is suggested that continued growth will drive up living standards for the populations as a whole.
Life expectancy is continuing to rise and infant mortality is falling. Access to education has improved, as has literacy.