Do I need a Tn visa for 6 weeks of work in the US?
 

As a Canadian, I will be traveling to the US for work every week for 6 weeks. Do I need to get a TN Visa? Or is a company letter sufficient?

If you're coming here to do business (business meetings, etc.) you're exempt because you are from a visa waiver nation.

If you come here to "work": Visa.

"All foreign workers must obtain permission to work legally in the United States."

"Nonimmigrants enter the United States for a temporary period of time, and once in the United States, are restricted to the activity or reason for which their nonimmigrant visa was issued."

http://www.uscis.gov/working-united-states/working-us

Some might argue this doesn't apply if you speak Spanish, but then we'd have to move the thread.

Why are you asking for legal/immigration-related advice relating to your job on an anonymous Internet forum? :confused:
Surely your company HR deals with this by providing you with the requisite paperwork and advice.

If you are going to be working for a US firm, paid in the US... then you will be working in the US.

If, however, you are going to be working for a Canadian firm, paid in Canada, and temporarily stationed in the US in order to conduct meetings, etc. then that's a different story.

There is a difference between visiting the US in order to work and visiting the US for business purposes.