Abstract: Synthetic source injection (SSI) is an extremely useful technique for the validation of photometric survey data. It consists in artificially inserting objects, such as galaxies and stars, with well-known properties into real astronomical images. By processing images with injected sources, we can obtain measurements of the observed properties of these sources and compare them to their true properties. The transfer function between observed and true values allows us to quantify the accuracy of image processing pipelines in recovering the real properties of targets in a survey. Given the high level of precision that current and future large-scale structure surveys are expected to achieve in constraining cosmological parameters, a robust characterization of uncertainties is necessary, in which SSI plays a critical role. Furthermore, SSI is helpful in simulating weak-lensing magnification effects, as well as in providing a way to infer the photometric redshifts of wide-field objects by extrapolating a color-redshift relationship from injected deep-field objects. The quantities derived from these applications are paramount to the cosmological analysis, making SSI an important part of the process. In this talk, I will describe Balrog, the SSI effort in the Dark Energy Survey (DES) Y6 cosmological analysis, in which we injected real sources from the deep fields over the entire 5000 deg^2 wide-field survey footprint. I will explain the process of injecting the sources and validating the injected sample, as well as the applications in the DES Y6 weak-lensing magnification and photometric redshift calibration projects. Finally, I will highlight ongoing work on SSI for the Legacy Survey of Space and Time (LSST), particularly on the recently released Data Preview 1 (DP1), as well as future potential applications.