Many mammalian cells operate via precise regulation of their internal electrical and chemical state. Understanding these dynamics is crucial for identifying how cells function in response to environmental stimuli, as well as how dysfunction leads to disease. Since a typical mammalian cell is only ten microns in diameter, a primary obstacle to probing its internal chemical and electrical activity is a lack of experimental interfaces that are both small enough to be minimally invasive and yet scalable enough to address many cells in parallel. In this talk, I will describe how vertical silicon nanowires can address this problem by providing a highly scalable chemical and electrical interface to the interior of living cells. Using this interface, we can deliver a diverse array of biomolecules across a cell’s membrane in order to perturb and study its internal biochemical environment. Moreover, we can use these nanowires as electrical probes to control and monitor the activity of neurons in an in vitro neuronal network. I will discuss examples of how these devices can provide new ways to reverse engineer biological circuits and uncover the cellular origins of disease.