request.headers["Content-Type"] # => "text/plain"
replace “Content-Type” with the name of the header that you want to read.
Update for Rails 4.2
There are 2 ways to get them in Rails 4.2:
Old way (still working):
request.headers["Cookie"]
New way:
request.headers["HTTP_COOKIE"]
To get a Hash with all headers of the request.
request.headers