Who wrote the book of Genesis? (Overview) I've heard reference in other questions on this site that seem to indicate a belief that Genesis was written by "the same man."  What do we actually know about wrote Genesis (whether a single, or multiple authors)?
There are two theories:


Traditionalists

Tradition says that Moses wrote the Pentateuch, including the book of Genesis.  

Here is a lengthy argument for this.
JEDP Theory

Some modern scholars believe that Genesis (along with the rest of the Pentateuch) was written by a series of authors.  The idea comes from the fact that God is referred to using multiple names (such as Yahweh, Elohim, Yahweh Eloheinu, etc.).

The idea is controversial based on (1) the fact that it goes against tradition and (2) the fact that even today we use multiple names for God ("our Heavenly Father", "God the Father", "God", "Jehovah", etc.).


Those are the two main theories.  Unfortunately, we don't have any way of truly knowing who wrote Genesis.  That's the reason for the varying theories.