Fish belonging to the Family Mugilidae, commonly known as grey mullet, are important in coastal water habitats by value of their unique role as detritivores feeding on resources not eaten by other competing fish groups and their importance in artisanal subsistence fisheries and aquaculture. This thesis contributes towards our understanding of the mugilid biology by studying the population biology of two species of grey mullet, the sub-tropical water abu mullet Liza abu (Heckel, 1843) in Central Iraq (the first study since 1996) and the temperate water thick-lip grey mullet, Chelon labrosus (Risso, 1827) in Northwest Wales (the first study in the UK since 1970). During the period between October 2014 and November 2015, 609 abu mullet were collected from three locations (Al-Najaf, Babylon and Karbala) in Central Iraq. Fish ranged between 85-220 mm total length, 13-110g wet weight and 0-5 years of age. Growth was negatively allometric (increase in length more than increase in weight), (b=2.35) and described by: Lt=25.4(1–e-0.24(t+1.57)). Monthly changes in condition factor (CF) and gonadosomatic index (GSI) indicated peak values occur in April-June coincident with the timing of the spawning period and confirmed by visual examination of gonad maturity status. During the period between January 2014 and December 2015, 373 thick-lip grey mullet were collected from various locations in Northwest Wales. Fish ranged between 373 – 680 mm total length, 555 g and 4.35 kg wet weight and 6-14 years of age. Growth was positively allometric (increase in weight more than increase in length), (b=3.32) and described by: Lt=83.9(1–e-0.08(t+0.79)). Monthly changes in condition factor (CF), body depth (BD) and gonadosomatic index (GSI) indicated peak values occur in October-December coincident with the timing of the spawning period and confirmed by visual examination of the gonad maturity status. Examination of gut contents for both species indicated a mixed diet, dominated by organic matter, algae and plant material. The suitability of various calcified structures (vertebrae, pectoral arch bones and opercular bones) as alternative ageing structures to scales/otoliths were examined. For both species, the best readabilities in all hard parts were obtained following simple cleaning, burning and coating with cedarwood oil. Statistical analyses indicated close similarity in ages in both species when aged using scales, otoliths or vertebrae and significant underestimation of age using pectoral arch and opercular bones.