The Organization for Economic Cooperation and Development (OECD) held a webinar on February 7, 2023, to present the methods described in Test Guideline (TG) No. 125 to determine the size and size distributions of nanomaterial particles and fibers spanning from one nanometer (nm) to 1,000 nm. It also presented the use and limitations of this TG, as well as of the validation exercise that was done to support its development. OECD organized the webinar to increase awareness of the newly adopted TG. The webinar is available for replay, and the presentations are available online.