US citizens are taxed by the IRS on “global accreations of wealth” no matter where or how you obtained it, however, there are a few exceptions. Your tax picture can be very different if you retire to France instead of the United Kingdom or Spain.

For example, the US and France have a tax treaty that says retirement payments such as Pensions will only be taxed “in the source state of income”.

Source: Article 18 of Convention between the Government of the United States of America and the Government of the French Republic

We recommend speaking with an attorney for offical advice on this matter.

A US citizen can theoritically live outside the US and never return while retaining their US citizenship.

Maintaining ties to the United States is essential if you wish to retain your US citizenship. This can include returning to the US every once in a while, filing a US tax return, and maintaining a US residence. Although you will want to plan your travel back to US very carefully if you want pass the “Physical Presence Test” and help make getting the Foreign Earned Income Exclusion easier by being outside the US for 330 or more days during any year.

For the most part, as long as the clients remain US citizens, we can continue to work with them. It also depends if you will become a "permanent resident" of that country.