Thin layers are water column structures that contain concentrations of organisms (or particles) that occur over very small vertical scales (a few meters or less), but with large horizontal scales (e.g. kilometers). Thin layers are now known to be common phenomenon in a wide variety of environments and can be a critical componant in marine ecosystem dynamics and functioning. While knowledge about their dynamics is important to our basic understanding of oceanic processes, thin layers can have significant impacts on both oceanographic and defense related sensing systems, e.g. thin layers can affect underwater visibility, imaging, vulnerability, communication and remote sensing for both optical and acoustic instrumentation. This paper will review the history of thin layers research, their ecological significance, innovations in oceanographic instrumentation and sampling methodologies used in their study, and the consequences of their occurence to oceanographic sensing systems.