It depends on what you want to do. Music Theory is a way to describe what you hear with words. There maybe parts in this theroy which will help you alot, there will be parts, which you think sound aweful and are useless. For me, it is really helpful now, after many years of "just doing it". It is not necessary for doing a good job or making music which you like. For example, when I did my first feature lenght film, I used just a very common 4-chord-progression in A-minor over the whole film, and you now what? People loved it. I didn't even think about it, didn't even knew the word "chord-progression". I just let the pictures and the emotions lead me to decide, which instruments, which tempi, etc... (And now when I think about it, I also didn't even used a reverb-plugin). It took me a very long time to finish that job and I was full of self-doubt and I was never sure, if the music is good or not. So, No, you don't need any theory. BUT After all this years, I love digging deep into theory right now. I really enjoy it. It is fun to learn some tricks which other composers before you found out about. I am still at the very beginning, but it definetly helps with composing (of course). An Example: I am sitting on a job right now. It is a very short film (3min) which had a dramatic change in the middle, and because learning about modulations technics in the last weeks I was able to write something satisfiying in a few minutes, without self-doubt, because I know it will work. Some theory-knowledge is so powerfull. This job will not give me any awards and it will maybe only be seen/heard by a very few people, but it feeds my family. And never learn theory without doing it. Don't read a 500 pages theory book without practising each page of it.