Explain the difference between systematic and unsystematic risk, and why one of these types of risks is rewarded with a risk premium while the other type is not.

Unsystematic, or diversifiable, risk affects a limited number of securities and can be eliminated by investing in securities from various industries and geographic regions. Unsystematic risk is not rewarded because it can be eliminated by investors. Systematic risk is risk that affects most, or all, securities and cannot be diversified away. Since systematic risk cannot be eliminated by investors it is rewarded with a risk premium. Systematic risk is measured by beta.