There is no reason that an English teacher must be a holder of B.Ed. degree or TESL Certificate. I have been a college teacher for many years, although my "original" specializations were Aeronautical Engineering (PhD), Mechanical Engineering (BS, MS) and MBA (International Management). During the past 35 years, I wrote countless number of articles, and correct the writings of hundreds of "American Engineers". I taught English at high schools as well as colleges in Taiwan and China, in addition to my lectures in engineering, design, manufacturing, and business management. Mad

It really depends on where you want to teach and at what level. Being certified will certainly make you more marketable but, perhaps more importantly, it will prepare you for the job. TESL/TEFL courses offer a great training opportunity at entry level. They can provide a thorough grounding in a new career and the right tools to approach a new profession.

Of course, one can improvise and become an English teacher simply on the grounds of being a native English speaker. But how would you like it if the person standing in front of you in a class is teaching you by trial and error, improvising their lesson as they go along and basically learning on the job at your expense. Surely you would prefer someone who has taken the time and the effort to prepare himself for the job ahead, learning which are the most effective teaching techniques, how to maximize classroom time, how to make his lessons useful yet fun, etc. Of course we all learn more from doing something than reading about it but having a solid understanding of what you are about to do can only be a good thing!

Finally, with regards to the need for a degree, this is often a requirement of the countries and their visa regulations and it is not so much a requirement of the institute that hires you. It is true that your degree does not need to be a B.Ed nevertheless you will need to hold a degree if you want to obtain your work visa and work legally. This is especially true in Asian countries.

I appreciate with you that B.Ed degree in not important for every teacher. Some teachers got teaching jobs without this degree. I think experience is most valuable than degree. My friend also got job in english school san diego.