This question starts to plague your mind the day you don the short white coat, and it never leaves until match day during your 4th year.  Although you don’t have to decide what you want to go into until the summer of your 4th year, it is a good idea to know what you would have to do to be competitive in a difficult specialty.  There are two great ways to obtain this information.

1. For 3rd and 4th year students, speaking to a student-friendly advisor is a great idea. HOWEVER, beware of the nice guy.  Find an attending who won’t be afraid to break your heart.  Better to have it broken now, than spend thousands getting your hopes up when you really had no chance anyway.
2. For first and second year students, the best resource in the world is the Outcomes of the Match materials provided by the NRMP.  The document is fantastic, but it is a lot to chew.  The link below is the most recent report, which summarizes the results from the 2009 match.

http://www.nrmp.org/data/chartingoutcomes2009v3.pdf

Periodically, I will be breaking down all the information provided by this document.  For the time being, look it over and study the graphs.  Everything you want to know about competitiveness of each specialty (not ophthalmology!) is in this document…here are a few to whet your appetite

Average Step 1 scores
Average Step 2 Scores
Average # matched applicants who were AOA
Average # or research projects
Percent of matched applicants with Ph.Ds