ABSTRACT
No matter how successful the standard model may be in describing properties of elementary particles, it is quite obvious that it cannot be the final theory of everything. For one thing, the standard model does not explain or describe gravitation: it ignores gravity for reasons explained in Ch. 1. Moreover, we know that the neutrinos have mass, a feature that was absent in the standard model. So, there is more than one reason to look beyond the standard model. There are various theoretical ideas on this general theme. In this chapter, we will try to give an outline to some such ideas. But first, we will discuss some other shortcomings of the standard model.
