The oil thickness information is essential for enhancing the effectiveness of oil-spill remediation techniques, such as mechanical skimmers, chemical dispersants, and in situ burning. Even though remote sensing methods can provide a global assessment of the spill extent, they are not suitable for providing accurate estimates of the local oil spill thickness due to varying oil properties and dynamic environmental conditions. To measure the thickness of oil spills locally and with high accuracy, in situ sensors were proposed in the literature relying on optical, electrical, acoustic, and vision sensing modalities. These sensing systems have different structures, including single probes, multielectrode arrays, and movable electrodes. This article provides a critical review of in situ oil spill detection (OSD) and thickness measurement systems. It focuses on explaining their working principles, properties, and limitations. The ultimate aim of this work is to facilitate the understanding of the state-of-the-art oil spill sensors and to provide an objective assessment of their ability to measure oil film thicknesses in open water environments. Despite its importance, this topic is poorly studied in the instrumentation and measurement domain. To the best of the authors’ knowledge, no similar reviews are available in the literature.