This special issue of Neuron continues our series on neurotechnology. In the first part of this series, we explored approaches to building and mapping neuronal networks. Now, we focus on the tools for reading the activity of these networks. The reviews and perspectives included here discuss methods used to measure neurophysiological data, ranging from the synapse to the whole organism. Some of the articles also describe integrated technologies employed to manipulate these networks—a theme that will be further developed in the next and final chapter of our special issue series.