Functional magnetic resonance imaging (fMRI) is a crucial tool for studying brain activity. It mostly uses the blood oxygen level dependent (BOLD) contrast to identify activated brain regions through neurovascular coupling. Therefore, fMRI alone does not provide direct information on neuronal activity or metabolic turnover. This limitation can be addressed by optical sensors, especially genetically encoded sensor proteins, which allow for the cell-specific detection of various biological signals, such as calcium, neurotransmitters, or metabolites. Combining fiber-based detection of signals from optical sensors with fMRI (ofMRI) can yield valuable insights into brain function and advance the understanding of neurovascular coupling, neurotransmitter regulation, or metabolic turnover. ofMRI may further be used to assess brain states or to clarify how anesthesia or drugs impact brain physiology. However, the integration of fiber-based optical recordings into fMRI poses challenges and creates pitfalls that must be avoided. This chapter describes the experimental procedures for acute simultaneous fMRI and optical recordings using genetically encoded sensor proteins, focusing on protocols for mice and rats. The complete experiment is divided into two parts: after the initial stereotaxic surgery for vector injection, several weeks for sensor protein expression have to be allowed. The actual experiment starts with a surgery for fiber implantation, immediately followed by ofMRI measurement. Protocols for resting-state and task fMRI are provided. The chapter aims to provide a practical guide for ofMRI, lists many potential pitfalls and details how these can be avoided. However, the chapter neither covers hardware details for optical recordings nor data analysis procedures.