Visits to within the Chernobyl Exclusion Zone are allowed, however a pass from the Ukrainian authorities is required before permission is granted. To obtain a pass, a copy of your passport must be sent to the authorities at least a week before entry into the Exclusion Zone, and the date of the day(s) of desired entry must also be specified. The 30 kilometre and ten kilometre exclusion zones are both guarded by soldiers of the Ukrainian Ground Forces, and a passport proving your identity must be shown to enter these zones. Upon leaving, all people and all vehicles are inspected by a Geiger counter, a device used to measure radiation levels.
There are several tour companies which organise tours within the Exclusion Zone, and the one I used on this occasion was the Ukranian Web tour. I can’t comment on the tours offered by other companies, but the Ukranian Web tour was excellent. The bus was small and personal; we were allowed as much time as we wanted and were never rushed; we visited an array of different sites within the Exclusion Zone; and our tour guide, Marek, had a clear passion for the history of the region and was very helpful and enthusiastic for the duration of the tour.