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In recent years aberrant maturity of sole (Solea solea) was recorded in WMR market samples, and during the North Sea beam trawl survey in August 2018 spawning female plaice (Pleuronectes platessa) was caught. These plaice were exceptionally skinny. Although the recordings could have been treated as incidents, it was decided to further examine these signals, and investigate if the maturation cycles in sole and/or plaice are changing. It was proposed to further look into the gonad development and spawning cycle of plaice and sole in the North Sea. As maturity can only be reliably studied macroscopically in the period directly prior to spawning and during spawning (ICES, 2018), microscopic maturity staging was used to reliably do year-round observations. The research question of this study is twofold, first focussing on the development of flatfish maturity throughout the year based on histological samples, and then checking if the observed development aligns with the expected development. Samples from fish were taken monthly during the regular market sampling, from April 2019 till March 2020. The fish were sampled from both the northern and the southern part of the North Sea. For plaice the border was set at 53°N and for sole at 52°N to separate the spawning components. For female fish statistical analyses were conducted to model the maturity over the months, correcting for some other covariates, using oocyte and egg proportion and diameter as the responses. No statistical models were run on the male specimen, as only the screening for presence or absence of sperm cell development stages was carried out. The development of cell types, especially the previtellogenic stage towards the vitellogenic stages is difficult to model. Thus it is hard to statistically underpin the maturation over time. This is largely due to the low number of data. The oocyte diameter increases during the maturation and following spawning cycle, from a small diameter after the previous spawning season, towards larger diameters as the nest spawning season comes closer. The diameters can reliably be modelled over time, for both plaice as well as sole. The expected development for both species is that the relative number of vitellogenic oocytes and their diameters increase towards the spawning season, and decline soon afterwards. As a consequence, the relative number of previtellogenic oocytes is expected to increase soon after the end of the spawning season, when the maturation cycle starts again. The plaice development seems to be aberrant from the expectation, especially in the southern North Sea. Plaice, as a capital winter spawner, is to be expected to build up the number of oocytes and let them evolve gradually towards the spawning season. The relatively high proportion of vitellogenic oocytes found from June till August, followed by the decline and an increase towards December is not in line with the expectation. The decline of the relative number of vitellogenic oocytes is most likely due to spawning activity, as post-ovulatory follicles were encountered in the samples from September to November 2019. Incidental spawning activity in summer for plaice in the southern North Sea is in line with the signals from the North Sea beam trawl survey. For sole no changes in the maturity pattern were found in this study, despite the signals from market sampling that sole gonads seemed to develop earlier in the season towards spawning than expected. The number of fish analysed in this study are low, due to the time-intensive labour of the histological analyses. Possibly the low number of samples missed any aberrant development in sole. However, analyses of gonadosomatic index and condition index (Fulton’s K) development over the year in the timeReport number 22.001 5 of 30 series from 1996 to 2019 did not show any changes over time (Chen et al. 2019, Wilkes 2020). This supports the results of the current study that no changes in gonad maturation are found.