Early Universe Cosmology and Tests of Fundamental Physics: Report of the P4.8 Working Subgroup, Snowmass 2001

This is the report of the {\em Working Group on Early Universe Cosmology and Tests of Fundamental Physics}, group P4.8 of the of the Snowmass 2001 conference. Here we summarize the impressive array of advances that have taken place in this field, and identify opportunities for even greater progress in the future. Topics include Dark Energy, Cosmic Acceleration, Inflation, Phase Transitions, Baryogenesis, and String/M-theory Cosmology. The introductory section gives an executive summary with six key open questions on which we can expect to make significant progress.