The hike to the top of Half Dome in Yosemite National Park includes a harrowing scramble up metal cables to complete the last 400 feet to the summit. Day hikers are required to have permits to use the cables, and the preseason lottery for permits is open now through March 31.
Make sure too that you're prepared to go on the days you request. Permits are good only on the dates for which they are issued. The cables go up around Memorial Day and remain in place through mid-October, depending on weather conditions. (Rain and snow can close the cables even if you have a permit.)
You'll receive notification starting in mid-April whether you were picked in the lottery. If you are picked, pay $8 per person to secure the permits and you're good to go. The National Park Service allows just 300 hikers per day on the cable portion of the Half Dome trail.
If you miss out in the preseason lottery, don't despair. The park awards 50 permits per day during hiking season via a second lottery. You apply at least two days in advance at Recreation.gov, and the same fees apply.
And remember, permits are required only for the cables. If you don't care about the last 400 feet to the top, enjoy the trail and watch others scramble up and down.