Neuromodulatory signals regulate neural circuit activity across multiple spatial and temporal scales, yet measuring them in vivo remains challenging. In this seminar, I will present two lines of work addressing this problem using complementary experimental approaches. First, I will describe work from my doctoral studies focused on the development of CNiFERs (cell-based neurotransmitter fluorescent engineered reporters), optical biosensors designed for real-time detection of neurotransmitter release in vivo. I will then focus on one of these sensors, namely the somatostatin SST CNiFER, which I used to demonstrate that somatostatin is not only a molecular marker but can also be released into the extracellular space by SST interneurons in the mouse cortex following optogenetic and sensory stimulation. Second, I will present recent postdoctoral work examining large-scale cortical dynamics in a mouse model carrying an autism-associated PTEN mutation. Using widefield calcium imaging simultaneously with behavioral monitoring in awake behaving mice, I measured cortex-wide neural activity in a variety of contexts, including spontaneous behavior, virtual reality navigation, and social interaction. These studies revealed increased long-range functional connectivity and altered representation of arousal-related behavioral states in cortical activity in Pten mutants. Together, these findings highlight how combining fluorescent calcium and neuromodulator sensors with systemslevel imaging can provide new insight into the role of neuromodulatory signaling in circuit function and dysfunction.