Since 2005, the National Institute of Standards and Technology (NIST) has collaborated with the National Institutes of Health (NIH), Office of Dietary Supplements (ODS) to improve the quality of measurements related to human nutritional markers of vitamin D status. In support of the NIH-ODS Vitamin D Initiative, including the Vitamin D Standardization Program (VDSP), NIST efforts have focused on (1) development of validated analytical methods, including reference measurement procedures (RMPs); (2) development of Standard Reference Materials (SRMs); (3) value assignment of critical study samples using NIST RMPs; and (4) development and coordination of laboratory measurement QA programs. As a result of this collaboration, NIST has developed RMPs for 25-hydroxyvitamin D2 [25(OH)D2], 25(OH)D3, and 24R,25-dihydroxyvitamin D3 [24R,25(OH)2D3]; disseminated serum-based SRMs with values assigned for 25(OH)D2, 25(OH)D3, 3-epi-25(OH)D3, and 24R,25(OH)2D3; assigned values for critical samples for VDSP studies, including an extensive interlaboratory comparison and reference material commutability study; provided an accuracy basis for the Vitamin D External Quality Assurance Scheme; coordinated the first accuracy-based measurement QA program for the determination of 25(OH)D2, 25(OH)D3, and 3-epi-25(OH)D3 in human serum/plasma; and developed methods and SRMs for the determination of vitamin D and 25(OH)D in food and supplement matrix SRMs. The details of these activities and their benefit and impact to the NIH-ODS Vitamin D Initiative are described.