The aim of this chapter is to give a brief overview of seven main modern innovation theories that are discussed in the other chapters of this book, to compare them and to reflect on their usefulness. These seven are the opportunity vacuum theory, the technological innovation systems theory, the political economy model of transport Innovations, the Multi-Level Perspective, Rogers' theory on diffusion of innovations, the hype cycle theory and a cluster of theories on the role of industries in innovations. The overarching feature of these theories is that innovations' success and failure is dependent on many factors: such as societal impacts, technical factors, user acceptance factors, firm related factors and factors related to political feasibility. Four applications of the theories are suggested in the chapter that can show the strengths and weaknesses within innovation systems and of specific transport innovation.