Hydroxypropyl Guar (HPG) is a polymer used extensively in the petroleum industry for fracturing oil and gas wells. Understanding the transmission of electromagnetic (EM) radiation in these fluids plays an essential role in the design, development, and improvement of optical sensing devices used to monitor and characterize these polymers. This investigation has resulted in the first comprehensive study of the transmission characteristics of HPG in the visible and near-infrared regions of the electromagnetic spectrum. Optical density data was collected for HPG fluids of various concentrations. Mixtures containing silica flour, a fluid loss additive, were also examined. Based upon optical transmission theory, a method was developed to determine the validity of calculating extinction coefficients at various fluid concentrations throughout the visible and near-infrared region of the spectrum. These coefficients were used to model the depth of penetration of EM radiation into HPG fluids. Calibration methods for optical density and concentration were also evaluated throughout the visible and near-infrared regions. Key results indicate that the best overall penetration into HPG fluids with no silica flour added occurs at wavelengths centered around 865 nm. In the case of fluids containing silica flour, the best penetration occurs at wavelengths centered around 1078 nm. These results, all with many others presented in this dissertation, will be of great utility in the design and development of in situ optical sensors for the characterization of HPG.