Carbon nanotubes exhibit exceptional properties in many different aspects. However, harnessing those properties in real applications is challenging mainly due to the structural heterogeneity, high impurity contents, and architectural defects that originate in the synthesis. In this dissertation, I aim to understand the nucleation and growth mechanisms of both the catalyst and carbon nanotubes in chemical vapor deposition processes and establish a relationship between the structure and properties of the synthesized carbon nanotubes. I will also discuss applications enabled by some of the nanotubes that I synthesized. First, I will discuss the structure control of vertically aligned carbon nanotube arrays to show that the nanotube diameter, density, and growth pattern are correlated with the migration and aggregation behavior of the catalyst across the substrate. Then, I will present experimental studies revealing new insights into the nucleation of the catalyst particles in the gas phase. Based on the new fundamental understanding of the nucleation and growth of both metal catalysts and carbon nanotubes, we have developed a new method to produce semi-aligned high-quality nanotube films, with a tunable number of walls, continuously at an ambient atmosphere with a record high production rate of 1400 m h-1. With this technique, we have reduced the catalyst impurity content and increased the production rate of the carbon nanotubes while simultaneously maintaining a high Raman G/D ratio of >70. The carbon-catalyst interaction during carbon nanotube growth is also studied by planting and etching the carbon nanotubes on a metal melt. We found that the carbon was readily removed by H2 from the growing front of the carbon nanotubes. Finally, we exploited the applications of these synthesized carbon nanotubes in achieving high power thin-film thermoacoustics, high power battery, and dynamic mechanical interface.