This informal session will discuss Wigner's quantum mechanics as a basis for understanding 
what a particle is and how to define its quantum numbers through the little group and measure its spin. 
After this introduction I will show the basic building blocks of the spinor-helicity formalism, an efficient 
formalism which has been used in the last 35 years to find remarkably simple expressions for scattering 
amplitudes. Finally, the link between the two topics will be shown. This informal session is designed 
to be at an introductory level, to be informal (and open to questions and discussions at any time) 
and somewhat interactive.