Aligned carbon nanotubes (CNTs) possess great potential for transforming the fabrication of advanced interfacial materials for energy and mass transport as well as for structural composites. Realizing this potential, however, requires building a deeper understanding and exercising greater control on the atomic scale physicochemical processes underlying the bottom-up synthesis and self-organization of CNTs. Hence, in situ nanoscale metrology and characterization techniques were developed for interrogating CNTs as they grow, interact, and self-assemble. This article presents an overview of recent research on characterization of CNT growth by chemical vapor deposition (CVD), organized into three categories based on the growth stage, for which each technique provides information: (I) Catalyst preparation and treatment, (II) Catalytic activation and CNT nucleation, and (III) CNT growth and termination. Combining all three categories together provides insights into building the process-structure relationship, and paves the way for producing tailored CNT structures having desired properties for target applications.