Is it appropriate for a prospective PhD student to contact phd students at a target school? I am a prospective phd student. I wish to know if it is fine to contact the phd students of my target schools?

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It is absolutely fine to contact current PhD students of programs and departments that you are interested in. On an average, I get about 1 email of this sort everyday and I make my best effort to reply intelligently.

Keep in mind though that many PhD students will simply not reply to your email. Sometimes folks are inundated by too many emails and commitments to respond to any additional emails and sometimes people just have bad emailing tendencies in general.