We study several classical duality results in the theory of tensor products, due mostly to Grothendieck, providing new proofs as well as new results. In particular, we show that the canonical mapping Y ∗ ⊗π X → (L(X, Y ), τ)∗ is not always injective, answering a problem of Defant and Floret. We use the machinery of vector measures to give new proofs of the dualitites (X⊗ε Y )∗ = N (X, Y ∗), whe...