Sofia has two underground lines which intersect in the city centre at the Serdika stop. One line runs from the East (Sofia Airport or Business Park Sofia) to the West (Lyulin), via the city centre while the other, shorter, line runs from South to North (Nadezhda). The underground is clean and efficient, though it does get crowded at peak times. Tickets cost 1.60 leva for a single journey and can be bought from the ticket machines at the entrance to the stations. An electronic card with 10 journeys priced at 12 leva (and rechargeable) is a better deal if you are a group (you just pass the card between you) or are spending more time in the city.
Bus, trolley bus and tram tickets can be bought for 1.60 lev from the machine just behind the driver's cabin. The ticket is only valid for the journey on that particular tram/bus. Daily, weekly and monthly travel cards can be bought from kiosks as can packs of 10 tickets, which need to be punched in the machine inside. The fine for not having a ticket is currently 20 leva and is payable on the spot.