In recent years, there has been an explosion of interest in entrepreneurship from both practical entrepreneurs and researchers. While theories are helpful for explaining business-driven activities in a startup, they are also valid in reasoning for the practical activities occurring in the entrepreneurial context. We believe that startups would benefit from the awareness of these entrepreneurial theories and the understanding of how they can be connected to decision making in both business and engineering perspectives. In particular, we want to focus on theories that are already used by practical entrepreneurs and their advisors. As an example, we have studied the Scandinavian entrepreneurial ecosystem. We selected six groups of theories that might be particularly relevant for the startup population, namely (1) core competence and resource-based view, (2) effectuation, (3) the fulfillment of entrepreneurial opportunities, (4) bricolage, (5) business model innovation, and (6) lean startup. In this chapter, we explain these theories including the ongoing research around them, the connections among these theories and how they can be applied in a real case study.