Live in Canada but Work in the USA

Greetings. I am an attorney, self-employed really and am currently working in the USA. In fact the office is listed in Massachusetts. I wanted to know about filing taxes. Where do I file? Am I going to get hit twice for taxes due? Not sure what to do? IF I live in Ontario do I file a MA tax return or an Ontario and US Federal Govt Return
Sincerely,

Chris

I am a Los Angeles based Attorney and CPA who handles cross-border US-Canadian tax matters. Feel free to reach me through this site if you need assistance with your US and/or Canadian tax filings.