Via the Karakoram Highway.
Hunza is just 100 KM drive from Gilgit, and most people arrive by road and it takes almost 2 - 3 Hours to reach Hunza from Gilgit. The main bus stand is on the KKH Aliabad. There are booking agents in town for long distance buses & jeeps along the KKH. The journey from Islamabad can take as long as 24 hours.
From Kashgar China there is a regular international bus service to Hunza via Sost crossing over the Khunjerab Pass about 5000 meter high. Across river Hunza at Sost, there is a village called Khuda Abad. People usually do not stop at Khunjerab Pass, they just carry on their journey to Sost with the bus. From Sost, you can do a number of activities around like trekking in the valleys, or drive to Hunza-Karimabad 2 hours, where the Baltit Fort is standing. The Khunjerab Pass is open from May 1st to Dec 30th, but closed in winter.
The international bus waits until there are enough people, which can take days as of 2012. An alternative option is to take a first bus to Tashkurgan, stay at Tashkurgan one night, the next morning go to the immigration to get a departure approval, and then take a second bus to Sost, then another one to Hunza.
By plane
Gilgit Airport IATA: GIL is small and has 45 minute flights to Islamabad on PIA
PIA offers regular flights of small 42 seater planes between Gilgit and Islamabad. All flights, however, are subject to weather clearance, and in winters, flights are often delayed by several days.