To realize carbon nanotube (CNT) as on-chip interconnect materials, the contact resistance stemming from the metal-CNT interface must be well understood and minimized. In this study, we compile existing published results and understanding for two metal-CNT contact geometries, sidewall or side contact and end contact, and address their key performance characteristics. Side contacts typically result in contact resistances > 1 kΩ, whereas end contacts, such as that for as-grown vertically aligned CNTs on a metal underlayer, can be substantially lower. The lower contact resistance for the latter is due largely to strong bonding between edge carbon atoms with atoms on the metal surface, while carrier transport across a side-contacted interface via tunneling is generally associated with high contact resistance. Analyses of high-resolution images of interface nanostructures for various metal-CNT structures, along with their measured electrical characteristics, provide the necessary knowledge for continuous improvements of techniques to reduce contact resistance. Such contact engineering approach is described for both side and end-contacted structures.