An overview is presented of how and where key ink components influence the jetting behaviour during printing. Physical properties alone are an insufficient guide to ink performance. Small variations in ink viscoelasticity influence the upstream flow dynamics and jet break-up mechanism downstream. Recently developed novel techniques capable of quantifying fluid rheology of low viscosity complex inkjet inks at conditions similar to those during printing are discussed which have not been possible until recently. These techniques provide useful tools to differentiate between apparently identical inks but which show different jetting behaviour. This is often the case, where ink batch variations or a minor formulation alteration causes different jetting behaviour and influence print reliability.