Orthodoxy believes in form of Apostolic Succession, as discussed here:
http://www.orthodoxanswers.org/answer.aspx?ID=3
Orthodoxy does not have concept of validity that is similar to the Roman Catholic concept, although some Orthodox bishops and synods have adopted uniform rules on the matter. The Patriarchate of Moscow, for instance, recognizes Roman Catholic orders and do not re-ordained Roman Catholic clergy who enter the Orthodox communion. On the other hand, in spite of occasional statements made at a time when the Anglican Communion was still mostly traditional and orthodox, Anglican ordinations are never accepted, which means that Anglican clergy entering Orthodoxy are received as laymen and require proper training and ordination to become Orthodox clergy.