Non-Christians and Christians who belong to churches without valid apostolic succession or valid sacraments (i.e. Protestants) may not receive Communion. This is because they do not meet the requirements for ecclesiastical community stated above. They do not have validly ordained ministers, nor valid sacraments, nor do they profess substantially the same faith as Catholics. (CCC 1400).
Eastern Orthodox may receive Communion in special circumstances, provided that the local bishops approves (CCC 1399).