This chapter presents an account of commonly accepted methods for measurement of the gill area of fishes, giving more emphasis to weightage of different parameters. The structural complexity of air-breathing organs of Indian fishes necessitates the application of stereological techniques. The methodology for morphometries varies in accordance with the structure and system to be measured. If the organ is of definite geometrical shape, its measurement is simple, but if the surface of the structure is complex and corrugated, measurement involves complex principles. The respiratory system of air-breathing fishes consists of gills and different types of accessory respiratory organs, which are very complex structures in configurations and as such, require special methods for measurement. The relationship between respiratory area and body mass was examined during development of water- and air-breathing organs of the fishes through the method of logarithmic transformations. The regression coefficients for filament numbers, filament length, and secondary lamellae are always higher for a water-breather than for an air-breather.