Most visitors enter Panjakent and the Zeravshan valley from Samarkand, which is just accross the border to Uzbekistan. You will need a valid Tajik visa to enter and a double/multi-entry Uzbek visa if you intend to return the same way you came. There is no public transport crossing the border and unless you have arranged your trip through one of the many Uzbek tour agents, you will have to switch taxis at the border. Taxis leave from Pandjakent Koutchasi, the south eastern part of the Registan. The trip to the border takes about 30 minutes and costs US$3 per person. From the border, the trip takes another 30 minutes and costs another US$3 per person. Travel agents at Samarkand organize the trip for about US$40 including transportation, guide, admission fee and "border crossing fee".
Warning: as of January 2012, the Samarkand-Panjakent border crossing is closed, with no signs of opening in the future.
From Khujand or Dushanbe, it is a spectacular but exhausting 7-10h trip to Panjakent in shared taxi. In 2009, the price for a seat is about 140TJS for the Dushanbe-Panjakent bit. The roads range from virtually absent to spectacularly good, depending on whether you travel on the original road or one of the bits already repaired.
The distance from Dushanbe is about 230 km. You will have to pass Varzob Gorge and go over the 3,370 m high Anzab Pass. In 2006, the 5 km long Anzab Tunnel has been opened. From Khujand, you will have to pass the Shakristan Pass with a height of 3,380 m.
Panjakent also has a airport from which small planes occasionally fly to Dushanbe. There is no schedule. Normally, if the passes are closed and enough potential travellers have assembled, Tajik air runs a trip or two.
