Every now and then, you run into one of THOSE problems - inexplicable, and something it seems no one else has solved before you. This afternoon, I hit one of these. Basically, I had just installed RVM, Ruby_1.8.7 and Rails_2.3.8 - everything was working great - I could run a server just fine, and use the running site. Eventually though, I found something broken. Easy enough to diagnose, I thought - I’ll just start up a debugging server, and work out whats going on. Wrong. Wrong, wrong, wrong. What followed was a continuation of this error: You need to install ruby-debug to run the server in debugging mode. So I had a Google - lots of people have run into this problem before (here and here specifically) - the problem is, most of these problems came down to a missing library called libreadline - for them, installing the library corrected all of their problems. In the end, I stumbled across this_post, from a guy having issues with Ruby after upgrading his Mac OS X - finally - he suggested something I hadn’t already tried. And, as it turns out - it worked! I have no idea why - but there we go. If you are having a problem getting Ruby 1.8.7 to acknowledge ruby-debug is there when running a debugging server, try this: gem uninstall ruby-debug gem install ruby-debug linecache –no-ri –no-rdoc script/server –debugger … It might just work for you!