Unhealthy dietary patterns are a major contributor to morbidity and premature death around the world, with the burden of disease from suboptimal diets continuing to rise. Contemporary dietary patterns are influenced by increasingly industrialised and globalised food systems that promote food environments overwhelmingly dominated by unhealthy foods and drinks manufactured, marketed and sold by large companies. Retail food environments are important to the perpetuation and exacerbation of this trend as these are the settings in which most people buy most of their food. In high-income countries, and increasingly commonly in middle- and low-income countries, supermarkets are the main source of foods. Various tactics are applied to shape purchases in this setting, and the ways in which the healthiness of purchases can traditionally be influenced in this environment are well-documented. Online grocery shopping is becoming increasingly popular in many parts of the world, although how retailer strategies are being adapted for online supermarkets, and the impact of their different systems and architectures on customer behaviour, is still not yet well understood. The opportunities to improve dietary patterns afforded by this online shift have similarly not been well explored to date. Through this thesis I aimed to assess how online supermarkets in Australia apply various strategies that may influence purchases, and to develop and evaluate an intervention to support healthier purchases in an Australian online supermarket. This thesis firstly investigates marketing strategies that are applied in online supermarkets to influence the healthiness of purchases. I found that there are myriad ways that retailers may attempt to influence purchases in online supermarkets. However, to date very few studies of their application had been published and few of the more sophisticated techniques that are available to retailers had been investigated. Through two subsequent studies of the online stores of the two dominant supermarket retailers in Australia, one focussed on the display of product information and the other on the prominent placement of products, I found that the provision of information to assist with healthy decision-making is currently limited and a large number of unhealthy products are displayed to shoppers. This thesis subsequently outlines the development, implementation and exploration of an intervention to support healthier purchases, tested in a real-world online supermarket setting. The intervention modified the user interface of Australia’s largest online supermarket retailer and interrupted usual purchasing paths with the aim of encouraging easy and sustainable changes to purchases. In-depth interviews with trial participants provided insights into the barriers and enablers to intervention adherence, as well as assisting with the identification of other factors that can influence the healthiness of purchases in online supermarkets more broadly. The findings suggested that this intervention could usefully support healthier decision-making in online supermarkets, however entrenched participant behaviours and attitudes as well as the structures and settings of the online supermarket may undermine these efforts. Taken together, this thesis identifies risks and opportunities offered by the shift to online grocery shopping. By exploring the strategies being used to influence purchases and an intervention to support healthier purchases and promote improved dietary patterns (despite the inexorable promotion of unhealthy products), findings from this thesis are relevant to public health policy, practice and research in these relatively novel but increasingly important food environments.