This thesis focuses on the link between condition, defined as the magnitude of fat reserves, and maturation in two sub-populations of Atlantic herring (Clupea harengus L.). Histological, fatty acid (FA), univariate and multivariate analyses were used in a multi-scale approach to elucidate the relationship between body fat and maturity. Furthermore, the accuracy of commonly used proxies of condition and maturity was tested. No evidence was found to support the hypothesis that a threshold of fat must be exceeded for first maturation to occur; however, a size threshold was observed. During maturation, herring may be capable of both selectively incorporating certain FA into the ovary and also of synthesising FA within the ovary itself. Mesenteric fat was highly dynamic during maturation and likely plays a role in gonad development. Commonly used morphometric condition indices were not related to mesenteric fat and the relationship between morphometric indices and other more direct indices was dependent on maturity stage. Macroscopic maturity staging was unreliable and errors led to an under-estimation of the herring spawning stock biomass of up to 26%. A gonadosomatic index was validated which was able to discern between immature, mature and recovering fish more accurately than macroscopic staging. Few differences were found between North Sea autumn-spawning (NSAS) and Norwegian spring-spawning (NSS) herring in this study. The FA profiles of both sub-populations were similar over the maturity cycle and the effects of length and maturity stage on mesenteric fat were analogous for both populations. A photoperiod cue of first maturation was found for Atlantic herring. However, this cue differed between the subpopulations, with NSAS herring maturation being triggered by the spring equinox and NSS herring maturation being triggered later. A multi-scale approach was successfully employed to demonstrate that there is an intrinsic link between fat reserves and maturity in herring.