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Type II supernovae (SNe II) are excellent tools for studying stellar evolution, mass loss, cosmology, and to probe possible explosion mechanisms. By examining the spectroscopic diversity of SNe II, this thesis aims to improve the understanding of SNe II classifications and their physical and progenitor properties. In this thesis, methods for characterizing the spectroscopic features of SNe II are updated and formalized. SNe II are well studied in the optical and have been found to be a diverse group. However, near-infrared (NIR) spectra of SNe II are rare. This thesis includes the largest NIR spectra data set of SNe II published to date. These NIR spectra are quantitatively measured and it is shown that there is a dichotomy in the NIR spectral properties of SNe II. This dichotomy is in contradiction with findings in the optical that suggest SNe II form a continuous group. Using principal component analysis (PCA), the NIR spectra are reduced from a multidimensional data set to a few components which describe the majority of the variations. With this tool, it is shown that the dichotomy seen in the spectral measurements is also a major source of variation, 6% of the total variation. In addition, two spectroscopic templates are created for SNe II in the NIR, which can aid in future cosmological studies. The presence, or absence, of carbon monoxide (CO) in the NIR spectra of SNe II is studied and found to have a possible correspondence with NIR subclass. A unique SN II, SN 2013ai, with a longer than normal rise to maximum luminosity and high expansion velocities is studied. SN 2013ai is examined in the optical and NIR and found to exhibit properties between a SN II and stripped envelope supernova (SESN). This supernova could be a possible link between SNe II and SESNe.