We show that, for a separable and complete metric space M , the Lipschitz-free space F(M) embeds linearly and almost-isometrically into 1 if and only if M is a subset of an R-tree with length measure 0. Moreover, it embeds isometrically if and only if the length measure of the closure of the set of branching points of M (taken in any minimal R-tree that contains M) is negligible. We also prove that, for any subset M of an R-tree, every extreme point of the unit ball of F(M) is an element of the form (δ(x) − δ(y))/d(x, y) for x = y ∈ M .