This technical appendix describes the methods used to estimate underreporting multipliers for passive and outbreak surveillance systems in the United States. For passive surveillance, the underreporting multipliers were calculated using ratios of active surveillance case counts (from FoodNet) to passive surveillance case counts (from NNDSS and COVIS). These ratios were used to create PERT probability distributions, which were then applied to estimate the total number of illnesses for pathogens with passive surveillance data. The multipliers were adjusted based on the type of pathogen (bacterial vs. parasitic) and the distribution of ratios. For outbreak surveillance, multipliers were calculated using ratios of active surveillance case counts (from FoodNet) to outbreak-associated case counts (from FDOSS). These ratios were also used to create PERT distributions, which were applied to estimate the total number of illnesses for pathogens with only outbreak data. The multipliers were not distinguished by pathogen type in this case. The data used for these calculations was primarily from FoodNet and NNDSS. The results showed that the observed ratios varied significantly, and the multipliers were adjusted to account for this variability. The final multipliers were chosen based on a compromise value that balanced the different estimates. The results showed that the mean ratio was around 25.6, with a PERT distribution that had a mean of 25.5. The multipliers were applied to estimate the total number of illnesses for each pathogen. The methods used in this appendix provide a framework for estimating underreporting in surveillance systems and can be used to improve the accuracy of disease burden estimates.This technical appendix describes the methods used to estimate underreporting multipliers for passive and outbreak surveillance systems in the United States. For passive surveillance, the underreporting multipliers were calculated using ratios of active surveillance case counts (from FoodNet) to passive surveillance case counts (from NNDSS and COVIS). These ratios were used to create PERT probability distributions, which were then applied to estimate the total number of illnesses for pathogens with passive surveillance data. The multipliers were adjusted based on the type of pathogen (bacterial vs. parasitic) and the distribution of ratios. For outbreak surveillance, multipliers were calculated using ratios of active surveillance case counts (from FoodNet) to outbreak-associated case counts (from FDOSS). These ratios were also used to create PERT distributions, which were applied to estimate the total number of illnesses for pathogens with only outbreak data. The multipliers were not distinguished by pathogen type in this case. The data used for these calculations was primarily from FoodNet and NNDSS. The results showed that the observed ratios varied significantly, and the multipliers were adjusted to account for this variability. The final multipliers were chosen based on a compromise value that balanced the different estimates. The results showed that the mean ratio was around 25.6, with a PERT distribution that had a mean of 25.5. The multipliers were applied to estimate the total number of illnesses for each pathogen. The methods used in this appendix provide a framework for estimating underreporting in surveillance systems and can be used to improve the accuracy of disease burden estimates.