Gravitational lensing of distant sources by galaxy clusters produces rare, strongly elongated images. The properties and statistics of these gravitational arcs are proxies for the abundance, internal structure and dynamical activity of clusters, as well as for cosmology and structure formation in general. I shall discuss some astrophysical implications of gravitational arcs and describe how arc statistics is at present believed to depend on the issues mentioned above. In addition, I will review the theoretical predictions available for arc statistics and how these compare with currently available observational data.