# In what sense are loop diagrams quantum corrections?

What's so not-quantum about tree-level diagrams?

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From the tag of the question, you are already assuming "FIELD" theory, are you? So the queston is how classical the fields are? –  user135 Dec 18 '11 at 16:34

The reasons were given here. Essentially, at tree level you recover classical results. Loop corrections are proportional to powers of $\hbar$ and these are quantum terms.
It is a bit more obscure for particles with spin, because then the classical result has still an $\hbar$. –  user135 Dec 18 '11 at 16:33