Grep through subdirectories

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http://askubuntu.com – Add a string to a text file from terminal I've been looking at this thread. The solution (number 2, with ls | grep) works perfectly for files called .txt in the current directory. How about if I wanted to search through a directory and the subdirectories therein? For example, I have to search through a directory that has many subdirectories, and they have many subdirectories etc. I'm new to Linux sorry, so I'm not sure if this is the right place (HowTos)