The accurate and reliable measurement of reproductive hormones and related analytes are essential for successful research throughout the Fertility and Infertility Branch (FIB) of the NICHD and beyond. The Ligand Assay and Analysis Core of the Clinical and Basic Studies in Polycystic Ovary Syndrome group has been providing this service for over two decades. This open-access core is critical for our and other NCTRI centers, as well as an array of individual and program-type awards made through FIB, broader NICHD and other approved users. The Core dedicates considerable effort to characterizing each assay and to continually reviewing important parameters for each test to maintain rigor, reproducibility and optimal assay performance. The Core has continually evolved, both implementing well-characterized, state-of-the-art assay methods and developing new in-house methodologies to meet the needs of investigators. The Core provides a centralized location with a highly-trained technical staff and the appropriate instrumentation to maintain exceptional standardized methods. This reduces unnecessary and expensive redundancy of effort by other investigators and provides the further advantage that individuals blinded as to the experimental details perform the analysis. The Core's emphasis on maintaining appropriate quality assurance measures ensures analyte data avoids assay reliability pitfalls that may compromise validity and the scientific interpretation of the findings. In addition to providing high-quality, cost-effective assay and analysis services, and developing new methods, the core provides assistance and consultation on assay methodology, data reduction/analysis and data interpretation to investigators that access our Ligand Core Facility. The utility and essential nature of this core to reproductive research is demonstrated by its contribution to 119 primary publications and four reviews since 2014.