Christiansøfarten operates passenger ferries to Christiansø from Gudhjem. From July to late August, ferries depart Gudhjem daily at 10am, 12.30pm and 3pm; return ferries depart Christiansø at 2pm, 4.15pm and 7.30pm. Sailing time is around an hour. Note that there are fewer sailings outside peak summer.
In a gorgeous gesture, 10am sailings in summer are serenaded from Gudhjem's harbour by the local choir.
From November to mid-April, there is a weekday service with the post-boat – check the website for details.
The main bus stop in Gudhjem is near the harbour. There are good bus links to other parts of the island.