The nucleus accumbens plays a key role in guiding motivated behavior by integrating glutamate inputs, the major driver of neuronal activation, and dopamine (DA) inputs, a critical source of neuronal modulation. Therefore, it is important to understand how glutamate and DA transmission interact, particularly in reward-seeking behavior. Substance use disorder (SUD) is a brain disease in which reward-seeking becomes dysregulated. In this dissertation, I use fiber photometry to extend our understanding of this interaction during two reward-seeking tasks.