This thesis first reveals the mechanism of Goertler instabilities and then demonstrates how transitions at hypersonic flows can be effectively controlled (either promoted or suppressed) with Goertler or Klebanoff modes. It focuses on understanding and controlling flow transitions from mild laminar to fully turbulent flows at high speeds-aspects that have become crucial at the dawn of an incredible era, in which hypersonic vehicles are becoming available. Once this occurs, it will be possible to travel from Beijing to Los Angeles within just 2 hours, and we will all live in a genuinely global village-and not just virtually, but physically. Goertler instabilities have often been used to promote flow transition in hypersonic vehicles. However, how Goertler instabilities are excited and how they evolve in hypersonic flows are questions that have yet to be answered.