Load and Resistance Factor Design criteria for steel building structures were developed from first-order (second-moment) probabilistic principles in a research project described in this paper. The underlying assumptions, the probabilistic basis, the calibrations process, and the final format are examined. Specifically, the paper concerns itself with the background and the choice of the particular first-order model, with the data bases used in evaluating the loading and the resistance functions, the choice of the safety index, and with the final selection of the load and resistance factors. Load combinations and serviceability as well as maximum strength criteria are tested.