This thesis explores how successful start-up innovators handle the early stage of web product development, contributing to academic knowledge on agile software development (ASO) and the fuzzy front end (FFE) of innovation. For exploration of the FFE behaviour, a multiple case study of 10 successful start- up innovators was conducted. Data from personal open-ended interviews with the innovators were analysed to uncover common themes. Results reveal that start-up innovators are agile in their product concept development, and use activities and process phases that differ from the existing FFE models. The participants created working prototypes, conducted alpha tests, and collaborated with customers to develop alpha-tested prototypes rather than written product concepts. The author developed an agile concept development (ACO) model to illustrate this concept. The research also unveils potential risks of ACO. A lack of systematic analysis, intuitive decision-making, and a missing business plan potentially increase the risk of commercial failure, while a poorly defined product concept and potentially.unsustainable development practices can lead to challenges in later stages of software development. Web product companies shall encourage ACO to create innovative web products. They should seek to attract innovators because they are the driving force behind the process. Implementing support systems in areas of low innovator attention and active portfolio management may lower the risk exposure of web product companies. More research on ACO, the characteristics of start-up innovators, and the early stages of other types of Internet products (e.g., mobile apps or games) must be conducted.