Fluid sensing techniques have been proposed by many research groups over the decades. These span a wide range of applications, which include industrial, medical and engineering fields. This review article focuses on the adopted sensing mechanisms in photonic devices for detecting fluids (gas/dissolved gas/liquid/electrolyte). A comparison between different technologies is made taking into account the performance indicators. The advantages and limitations of each of the techniques are highlighted, which will pave the way for future research and development in this area.