Tacttoo is a feel-through interface for electro-tactile output on the user’s skin. Integrated in a temporary tattoo with a thin and conformal form factor, it can be applied on complex body geometries, including the fingertip, and is scalable to various body locations. At less than 35μm in thickness, it is the thinnest tactile interface for wearable computing to date. Our results show that Tacttoo retains the natural tactile acuity similar to bare skin while deliver- ing high-density tactile output. We present the fabrication of customized Tacttoo tattoos using DIY tools and contribute a mechanism for consistent electro-tactile operation on the skin. Moreover, we explore new interactive scenarios that are enabled by Tacttoo. Applications in tactile augmented reality and on-skin interaction benefit from a seamless augmentation of real-world tactile cues with computer-generated stimuli. Applications in virtual reality and private notifications benefit from high-density output in an ergonomic form factor. Results from two psychophysical studies and a technical evaluation demonstrate Tacttoo’s functionality, feel-through properties and durability.