  Blue straggler stars are the most prominent bright objects in the
colour-magnitude diagram of a star cluster that challenges the theory of
stellar evolution. Star clusters are the closest counterparts of the
theoretical concept of simple stellar populations (SSPs) in the Universe. SSPs
are widely used as the basic building blocks to interpret stellar contents in
galaxies. The concept of an SSP is a group of coeval stars which follows a
given distribution in mass, and has the same chemical property and age. In
practice, SSPs are more conveniently made by the latest stellar evolutionary
models of single stars. In reality, however, stars can be more complicated than
just single either at birth time or during the course of evolution in a typical
environment. Observations of star clusters show that there are always exotic
objects which do not follow the predictions of standard theory of stellar
evolution. Blue straggler stars (BSSs), as discussed intensively in this book
both observationally and theoretically, are very important in our context when
considering the integrated spectral properties of a cluster, or a simple
stellar population. In this chapter, we are going to describe how important the
contribution of BSSs is to the total light of a cluster.
