  Form factor axioms are derived in two dimensional integrable defect theories
for matrix elements of operators localized both in the bulk and on the defect.
The form factors of bulk operators are expressed in terms of the bulk form
factors and the transmission factor. The structure of the form factors of
defect operators is established in general, and explicitly calculated in
particular, for the free boson and for some operator of the Lee-Yang model.
Fusion method is also presented to generate boundary form factor solutions for
a fused boundary from the known unfused ones.
