in this talk, we give a brief introduction to a natural generalization of groups, called n-groups.\\ just as discrete groups represent the homotopy types of acyclic spaces, n-groups realize homotopy types of connected topological spaces $x$ such that $\pi_i(x)=0$ for $i>n$. in this talk, we adopt the formalism of simplicial sets, and define n-groups as simplicial sets satisfying certain a filling condition (introduced by duskin).\\ in the first part of the talk, we explain what a 2-group look like: this material is contained in any textbook on simplicial sets. we indicate how 2-groups arise in topological quantum field theory.