Flavors and Horizontal Symmetries

After a brief introduction to what are the basic flavor questions to be addressed, I introduce the underlying ideas of horizontal symmetries, with group $G_H$. For the purposes of specific model building, it is useful to classify models according to the scale at which $G_H$ is broken. I consider the three cases: below $m_t$; somewhat above $m_t$; and at $M_{GUT}$. After a discussion of the shadow sector in the $E_6 \times E_8'$ superstring, there is a summary.