How a Halakhah becomes Accepted

Any post-Biblical Halakhah is the result of a Rabbinical decision. Until about 1000 years ago there were Rabbinical courts whose authority was generally accepted — for example, the Sanhedrin in Israel or the Yeshivos in Babylonia. Decisions made since that time may be accepted only by some communities; for example, Sephardic Jews accept the rulings of R. Yoseph Karo in the Shulchan Aruch, whereas Ashkenazim accept the rulings of R. Moshe Isserles (the Rema).