Breaking the first tablets
After seeing that the Israelites had gone astray during his absence and his brother Aaron had made the Golden Calf, Moses broke the tablets (Exodus 32:19).
Second set of Tablets
God subsequently commanded Moses to carve two other tablets like the first (Exodus 34:1). In Exodus 34:27-28 Moses was commanded to recreate the tablets, and to rewrite the commandments himself. In Deuteronomy 4:13, 5:18, 9:10, God himself appears as the writer.
This second set, brought down from Mount Sinai by Moses (Exodus 34:29), was placed in the Ark, also known as the Ark of the Covenant (Exodus 25:16, 25:21, 40:20), hence designated as the "Ark of the Testimony" (Exodus 22:16, Numbers 4:5; compare also 1 Kings 8:9).
Various theories have been advanced as to why the text in Deuteronomy differs on some points with the text in Exodus (see below).