Thanks!

Interesting Question:

My job is to supply linguists to the US and Iraqi govt. ESL is a priority here. I cannot tell you the civilian side of the market/demand but on this side the Defense Language Institue does provide certification.

Last, it is shown (so far) that someone who's primary tongue is not English makes the best ESL teacher. This like any theory can be proven one way or the other. What are you planning? If you want to make this a career/living you might seek the certification.

Hope this helps.

It depends where you are. In California, I believe you need a a TESOL certificate to teach ESL to adults or equivalent experience at the adult education level. To teach at a community college you need a Masters degree in TESOL.