Early studies with protein transduction domains (PTDs), such as those derived from Tat and Drosophila Antennapedia, showed rapid, receptor- and endosomal-independent uptake of conjugated biological tools into all cell types. However, recent mechanistic studies suggest that these observations were artefacts of the positively charged nature of PTDs and that uptake is instead via electrostatic binding to the plasma membrane and subsequent endocytosis. Given these observations, we assess the future utility of PTDs for in vitro and in vivo cellular delivery.