Fluidity is about materials in motion - materials active and activated, mobile and mobilised. It relates to manifold entanglements and mixtures of actual materials rather than to a general flux of matter. Moreover, it involves rhetorical tropes, material imaginations, and theoretical challenges. Fluidity is an intricate phenomenon: often messy, viscous or turbulent, and sometimes disastrous. This volume contributes to a critical cultural rheology examining the material complexities, epistemic functions, political ramifications, and ecological dimensions of fluidity.