The National Park Service has initiated a permit/quota program for Half Dome. If you are planning to hike Half Dome during a summer weekend (Friday, Saturday, or Sunday) you better plan ahead because there will only be 400 permits handed out per day. Permits must be reserved in advance and will not be available in Yosemite. Backpackers (with a proper wilderness permit) will be able to get a permit to Half Dome with no additional reservation. You can find out more information here.