In this study, spatial and temporal variation in fat content and temporal variation in maturation of North Sea herring were examined. Fulton’s K, a widely used condition index, was proven to be an inconsistent measure of individual herring fat content, whereas relative condition factor is a more accurate measure.  The Distell fish fatmeter was shown to be an accurate measure of individual herring muscle fat content and was used throughout the study. Individual herring biological and fat data were collected during the annual acoustic North Sea herring survey in 2006 – 2007. Analyses of these data showed that immature herring in the northern North Sea were larger and had higher fat contents than those in the south, in 2006.  This spatial gradient may be due to spatial variability in temperature and food abundance. Individual-level analyses using three years of field data demonstrated that North Sea herring must reach a length threshold of approximately 250 mm before the onset of maturation can occur.  However, stock-level analyses using data provided by the ICES herring stock assessment, indicated that maturation of North Sea herring is not affected by density-dependent factors or by mean North Sea surface temperatures.  This study demonstrates that the accuracy of different measures of condition should be tested before being used in research and that spatial and temporal variability in fat content should be considered when conducting studies on fish condition and other life history parameters.