The Northern Lights collaboration for better 2-photon probes will combine the expertise of four very different investigators and laboratories from Montana and Canada with complementary interests and skills to tackle a fundamental problem in imaging of the activity of the living brain. It is now well agreed that genetically encoded, fluorescent biosensors are uniquely suited to provide the specificity and versatility necessary to image the activity of defined cells in the brain. It is also clear that to image cells in thick slices or whole brains it will be necessary to turn to 2- photon (nonlinear) imaging, which penetrates deeper into living tissues and causes less damage. Furthermore, 2-photon signals provide additional molecular-level information that 1- photon signals lack. Many years have gone into developing better fluorescent proteins and biosensors for linear imaging. Unfortunately, directly optimizing probes for the 2-photon nonlinear properties has been beyond the reach of biosensor engineers because of the overwhelming complexity of the current state-of-the-art 2-photon characterization. This is a critical impediment for the BRAIN initiative, because robust 2-photon probes with high brightness and stability are needed. For this project, the team will develop a new process and new instruments using available ultrafast lasers and advanced experimental designs that will empower biosensor engineers involved in the BRAIN project to evolve the next generation of multi photon probes.