We introduce an appropriate notion of inner amenability for locally compact quantum groups, study its basic properties, related notions, and examples arising from the bicrossed product construction. We relate these notions to homological properties of the dual quantum group, which allow us to generalize a well-known result of Lau-Paterson [46], resolve a recent conjecture of Ng-Viselter [51], and prove that, for inner amenable quantum groups G, approximation properties of the dual operator algebras can be averaged to approximation properties of G. Similar homological techniques are used to prove that ℓ 1 (G) is not relatively operator biflat for any non-unimodular discrete quantum group G; a unimodular discrete quantum group G with Kirchberg's factorization property is weakly amenable if and only if L 1 cb ( G) is operator amenable, and amenability of a locally compact quantum group G implies Cu( G) ∼ = L 1 ( G) ⊗ L 1 ( G) C0( G) completely isometrically. The latter result allows us to partially answer a conjecture of Voiculescu [67] when G has the approximation property.2010 Mathematics Subject Classification. Primary 22D35; Secondary 46M10, 46L89.