This presentation is intended to provide background on the principles involved in geotechnical and infrastructure applications of Time Domain Reflectometry (TDR). TDR is basically radar in which a  voltage pulse is launched along a coaxial cable. A reflection of the voltage pulse occurs at every location where the cable is being deformed and each location is distinguished by the reflection travel time. For example, localized shear in a rock or soil mass will deform a cable grouted into a borehole and the TDR reflection magnitude is proportional to the magnitude of cable deformation. In one variation of this principle, a hollow coaxial cable can be installed in a monitoring well and a reflection will occur at the air-water interface. This strong reflection makes it possible to monitor changes in water level. In another variation, parallel rods can be embedded in soil and TDR is used to measure travel time of pulses reflected from the ends of the rods. The voltage pulse travel time is proportional to the dielectric constant of soil which is heavily influenced by changes in free-water content and makes it possible to monitor changes in water content using TDR. This paper presents a summary of the principles involved in these various applications and a companion paper presents case histories of infrastructure monitoring. This paper begins with a discussion of pulse testing then defines transmission line parameters pertinent to TDR technology. This is followed by defining the TDR reflection coefficient and how it is used to measure cable deformation. Finally, the paper discusses measurement of pulse propagation velocity and how it is used to measure changes in soil water content.