US is one of the only two jurisdictions in the world which levy tax based upon citizenship as well as residence. If you are a US citizen or a green card holder, it does not matter where you stay in the world. You are liable to file and pay US taxes. We do not just file taxes, but we also help you to organize the affairs in such a way that the tax liability and social security payments are optimal. Further, if you have been outside of US and are not compliant with the US tax laws, we can help you by becoming compliant under streamlined offshore procedures. It can save you from never ending past tax liability and can make you compliant.  

If you are a US expat staying in India, we are even better placed with our understanding of your tax position here in India as well as USA.