Monitoring cellular activity at the stimulation site is necessary to confirm the illumination effect on the targeted population of neurons. Brain activity can be monitored in various ways. Optical functional imaging, functional magnetic resonance imaging, and behavioral assessments are examples of possible readout strategies.7 However, one of the most popular methods used in neurosciences research remains electrophysiology. It offers a high-temporal resolution and can probe activity at the cellular or network levels. Electrodes can be implanted and used for several weeks or months. Unlike fluorescent measurements using calcium or voltage fluorescent sensors, electrophysiological recordings come at virtually no cost; no dye introduction or transfection is needed. Extracellular recording is noninvasive to the cell, minimally fade with time (unlike photobleaching of fluorescent dyes), and is nontoxic (no photochemistry involved). It is thus, not surprising that this modality is often preferred over others. However, it is important to note that electrode implantation causes a certain amount of tissue and vascular damage, and they are susceptible to photo-electric artifacts (discussed in Sec. 4.2), to cross-talk between recording channels and signal degradation over long periods of time.82 Therefore, the right electrode assembly is primordial. By varying electrode types and frequency bandwidth, one may choose to record single units, multiunits, local field potentials, corticograms or even electroencephalograms.83 The need for combined optical light guides and electrodes (here referred to as optrodes) was present in the field of neurosciences before the beginning of optogenetics and preliminary designs of optrodes were made in the past for different applications.84,85 A major application for these early optrodes was simultaneous electrophysiological recordings and fluorescence collection. For example, Duff Davis, and Schmidt84 combined extracellular recordings to calcium-sensitive fluorescence measurement in awake animals using optrodes. Original optrode designs simply combined a commercial optical fiber to a commercial extracellular electrode.7,72Figure 3(a) shows a micrograph of such an optrode. This is a flexible, simple, and low-cost design that can easily be integrated in any electrophysiological recordings setup. In most cases, researchers choose conventional recording modes such as extracellular single-or multiunit or local field potential recordings, but other types of recording electrodes, such as ion-sensitive electrodes, can be used.86 An alternative to optical fibers for light delivery is to use a glass pipette recording electrode as the light guide. This is obviously not an optimal waveguide, but optical losses can be compensated by a higher input power. The pipette can thus serve simultaneously as an electrode and an illumination channel. Note that optical losses will occur along the shaft of the tapered electrode and a metal coating should be used to confine illumination at the electrode tip. Along this line, recently, Katz et al.87 presented the “optopatcher” that could potentially be used for in vivo patch clamp.