Understanding the processes that determine the spread of non-native species (NNS) is critical if the ir impacts on native species biodiversity and ecosystem processes are to be mitigated. The secondary spread of NNS may be controlled by a variety of factorsincluding their dispersal potential, the availability of suitable habitats, and their ability to cope with biotic and abiotic conditions in the novel environment. This thesis investigates patterns of distribution and abundance of the non-native gastropod Crepidula fornicatain Welsh coastal waters. In a combination of field and laboratory observations of larval supply, larval settlement and post-settlemen t processes, combined with work on limiting factors such as low temperature, I investigated factors controlling its current adult distribution and potential for further northward colonisation from its current northernmost Welsh population, the Milford Haven Waterway (MHW).Results of this research project showed that C. fornicatais well established in the MHW, with locally superabundant aggregations and no indication for reduced reproductive success. It occurs across a variety of habitat types and the availability of certain hard substrata wasfound to most likely even facilitate population establishment. This indicates that limited habitat availability and decreased reproductive potential due to the exposure to sub-optimal seawater temperatures unlikely explain its absence from the coastal waters of Mid and North Wales. Benthic recruitmentin the MHW,onthe other hand, was generally low and occurred during a much shorter time period compared to the long larval season, indicating that settlement and post-settlement processes may be highly important in controlling adult distributional patterns. Early post-settlement mortality (EPSM) is likely important in determiningpatterns of adult distribution, whils tlarval supply and associative larval settlement seem to beof minor importance. However, my results apply onlyto the distribution of adults in the intertidal, where exposure to harsher environmental conditions probably results in higher EPSM. Lastly, I found that the availability of certain microhabitats might attenuate the high levels of EPSMin the intertidal, thus having considerable impacts on fine-scale adult distributional patterns.The supply of late-stage larvae,in combinationwith hydrodynamic conditions and larval settlement behaviour,however, seems to be most important in limiting population spreadat a regional scale,due to the likely presence ofsubtidal populations. Thisshowsthe importance of incorporating settlement and post-settlement processes into studies on recruitment success when aiming to predict the potential spread of a potentially harmful invader.