NHD projects begin at your school, whether public, private, parochial, charter, or homeschool. From there, your project might be selected to advance to either a regional or affiliate (state, territory, or country) contest.
At an NHD contest, students present their work to judges, who give all students feedback on their work and select projects to advance to the next level of the competition. Between contest levels, students are encouraged to revise their project based on what they have learned and the judges’ feedback. Students can make any revisions that they want, in accordance with the Contest Rule Book.
The top two entries in each category at the affiliate contests are invited to attend the NHD National Contest. About 3,000 students from across the United States and overseas compete each June at the University of Maryland, College Park.
From The NHD Website