This thesis concludes a wonderful four-year journey at USC. I would like to take the chance to express my sincere gratitude to my amazing mentors and friends during my Ph.D. training. First and foremost I would like to thank my adviser, Prof. Fei Sha, without whom there would be no single page of this thesis. Fei is smart, knowledgeable, and inspiring. Being truly fortunate, I got an enormous amount of guidance and support from him, financially, academically, and emotionally. He consistently and persuasively conveyed the spirit of adventure in research and academia of which I appreciate very much and from which my interests in trying out the faculty life start. On one hand, Fei is tough and sets a high standard on my research at “home”— the TEDS lab he leads. On the other hand, Fei is enthusiastically supportive when I reach out to conferences and the job market. These combined make a wonderful mix. I cherish every mind-blowing discussion with him, which sometimes lasted for hours. I would like to thank our long-term collaborator, Prof. Kristen Grauman, whom I see as my other academic adviser. Like Fei, she has set such a great model for me to follow on the road of becoming a good researcher. She is a deep thinker, a fantastic writer, and a hardworking professor. I will never forget how she praised our good work, how she hesitated on my poor