I applied online. The process took 4 months. I interviewed at Caltech (California) (Pasadena, CA) in Apr 2011
Interview
Hard.... um.... you have to have some luck... Of course your GPA should be top in your school. Also you need to have a lot of other skills like you have won prizes or you have published papers or you have patents...
I applied online. The process took 1 week. I interviewed at Caltech (California) (Pasadena, CA) in Mar 2012
Interview
I really had no formal interview. I sent my CV by email to the Professor I was interested in and he answered me after 1 week that he was also interested. After that I applied to the graduate program at Caltech and got accepted.
I applied online. The process took 2 days. I interviewed at Caltech (California) (Pasadena, CA) in Feb 2012
Interview
Not all fields have interviews, but for mine it did. They fly you in with other candidates and you meet all the professors and learn about their work. It was a very relaxed feeling but definitely be prepared to tell them about your prior research experiences. Have a few questions to ask about their current projects as well.