  This is an overview on the {source operator method} which leads to the
construction of symmetry breaking differential operators (SBDO) in the context
of tensor product of two principals series representations for the conformal
group of a simple real Jordan algebra. This method can be applied to other
geometric contexts: in the construction of SBDO for differential forms and for
spinors, and also for the construction of Juhl's operators corresponding to the
restriction from the sphere $S^n$ to $S^{n-1}$.
