No, you don't need to pre-book; you can just turn up. You can visit the shelter 7 days a week from 10:00-19:00 in the summer and from 10:00-17:00 in the winter. When you arrive at ARK, walk up to the Olive tree with the tables and chairs underneath and find Marina or Joyce and introduce yourself. If you feel more comfortable letting ARK know you will be coming to volunteer, you can get in touch: doris.ark@hotmail.com (Greece) or contact us here in the UK.