With the advent of limit states design methodology in North American design specifications, there has been increasing demand to obtain statistical data to assess the reliability of structural and geotechnical designs. Reliability depends on load and resistance factors that are determined through calibration procedures using available statistical data. This Circular describes methodologies that can be used to determine load and resistance factors for geotechnical and structural design. The Circular begins with basic reliability concepts, continues with detailed procedures that can be used to characterize data to develop the statistics and functions needed for reliability analysis, presents detailed step-by-step examples, and concludes with practical considerations when statistical data are limited. Closed-form solutions for estimating load and resistance factors that can be used for simple cases, as well as more rigorous probabilistic analysis methods such as the Monte Carlo method, are discussed in detail. Procedures are provided for situations where either single or multiple loads must be considered. An example is also provided that demonstrates the effect of considering only the variability of the input parameters for a given design methodology versus considering the overall variability of the design method. Such an approach can also be used to assess the effect of variability of a given design parameter on the reliability of the design.