By ship
M/s Runeberg (http://www.msjlruneberg.fi/) cruises from Helsinki to Porvoo between May and September three to five times a week, departing at 10 AM and returning at 4 PM. The trip takes 3.5 hours one way and costs €25.00/36.00 one-way/return, half price for children. On Saturday, you can also opt to take the boat one way and the train back. The boat leaves from Linnalaituri on Helsinki's Market Square, opposite the President's palace.
By train
There is no regular passenger train service to Porvoo, but the Porvoo Museum Railways (http://www.helsinkiww.net/pmr/) run a vintage 1955 Lättähattu "Flat Hat" Dm7 from Kerava to Porvoo and back on Saturdays in July/August only. The trip takes 1.5 hours and costs €15/25 one-way return for adults, half price for children, no reservations, cash only. As of 2009, the train leaves Kerava at 12:10 and sets off back from Porvoo at 4:00 PM. Kerava, in turn, can be reached in 20 min by regular commuter train from Helsinki's Central Railway Station.
By bus
There are buses from Helsinki's Central Bus Station to Porvoo every 15-30 minutes. Standard/express services cost €10.30/13.20 one-way and take 55/65 minutes, so the surcharge is hardly worth it — the expresses just stop in Porvoo on their way to points further east.
By car
Porvoo is easily accessed via the E18 expressway east from Helsinki towards Kotka and the Russian border. The other option is the old Porvoo road, Highway 170, but it's considerably slower and not particularly scenic.
By bike
Bike fans may want to consider pedalling the 78 km along the scenic King's Road (http://www.kuninkaantie.net/) from Helsinki to Porvoo, or 50 km along the more direct Highway 170.