"Fusion" study designs combine data from different sources to answer questions that could not be answered (as well) by subsets of the data. Studies that augment main study data with validation data, as in measurement-error correction studies or generalizability studies, are examples of fusion designs. Fusion estimators, here solutions to standard stacked estimating functions, produce consistent answers to identified research questions using data from fusion designs. We describe a pair of examples of fusion designs and estimators, one where we generalize a proportion to a target population, and one where we correct measurement error in a proportion. For each case, we present an example motivated by HIV research and summarize results from simulation studies. Simulations demonstrate that the fusion estimators provide approximately unbiased results with nominal 95% confidence interval coverage. Fusion estimators can be used to appropriately combine data in answering important questions that benefit from multiple sources of information.