The Chinese people have a long history of migrating overseas. The overseas Chinese of today can be dated back to the Ming dynasty. When Zheng He became the envoy of Ming, he sent people to explore and trade in the South China Sea and Indian Ocean. Many of them were Cantonese and Hokkien. A large portion stayed and never returned to China.  Physical evidence such as Bukit Cina in Malaysia seems to indicate permanent settlements.
In 19th century, the age of colonialism was at its height and the great Chinese Diaspora began. Many colonies lacked a large pool of laborers. Meanwhile, in the provinces of Fujian and Guangdong in China, there was a labor surplus due to the relative peace in the Qing dynasty. The Qing government was forced to allow its subjects to work overseas under colony powers. Many Hokkien chose to work in Southeast Asia with their earlier links starting from the Ming era, as did the Cantonese. For the countries in North America and Australia, great numbers of laborers were needed in the dangerous tasks of gold mining and railway construction. With famine widespread in Guangdong, this attracted many Cantonese to work in these countries to improve the living conditions of their relatives. Some overseas Chinese were sold to South America during Punti-Hakka Clan Wars in the Pearl River Delta in Guangdong.
With the completion of railways, many overseas Chinese suffered from racial discrimination in Canada and the United States of America and they were barred from entering the country.
After World War II, the last years of the Chinese Civil War increased Chinese suffering. Some educated overseas Chinese did not return to the country as the condition deteriorated. http://www.creadersnet.com/newsViewer_english.php?id=666672