America is a unique country in that individual taxpayers are liable to report worldwide income on their returns, usually regardless of the source of said income or any other factors. Additionally, the United States imposes strict reporting requirements for Americans who hold assets in accounts overseas. 

There are exclusions and credits that limit, and in many cases, eliminate income tax liability for expats. These individuals need a practitioner with the know-how to keep them current on their filings to avoid any potential issues down the line.