Interferometry is a technique that uses the interference of waves to make precise measurements. The wavelength of the interfering waves acts like the tick marks on a ruler for measuring distance. Optical interferometers can make very precise measurements because the wavelength of the light waves they use is small — around one micron for instruments like LIGO and LISA. A fundamental limitation of interferometry is that precision of the measurement is limited by the stability of the waves used in the interferometer. For an optical interferometer, if the wavelength of the light fluctuates, a spurious signal will be generated that mimics physical motion. One way to mitigate the effect of a fluctuating source is to compare pairs of distances using a common light source. This is the underlying concept of the Michelson interferometer that was used by Albert Michelson and Edward Morely to search for the “luminferous aether” in the late 19th century. LIGO uses the same concept in its interferometers over a century later. In order for this technique to work, the lengths of the light paths must be precisely matched. While LISA’s orbits produce approximately-equal arms, they differ by up to a percent and fluctuate by almost the same amount over long time periods due to orbital mechanics. Time Delay Interferometry (TDI) is a technique that was developed in the late 1990s and early 2000s to allow LISA to take advantage of the “common mode rejection” effect despite having unequal arms. TDI takes advantage of the fact that LISA measures the interference in each one-way laser link individually. While each of these signals is dominated by fluctuations in the LISA laser wavelength, those same fluctuations are measured at multiple points in the LISA constellation with varying time delays. By combining these individual measurements and correcting for the time delays, and adding in some rough knowledge of the constellation geometry, a significant amount of suppression of laser wavelength noise can be achieved. The ability to suppress laser wavelength noise through TDI is primarily determined by the precision of the individual interference measurements and the accuracy of the estimates of the LISA arm lengths. TDI has been extensively examined in analytic studies, numerical simulations, and experimental analogues and has been demonstrated to work as expected. The LISA team continues to refine our understanding of this important technique to ensure that it will provide the sensitivity that LISA requires to achieve its science goals.