  The purpose of this article is to explore the properties of integrable,
purely transmitting, defects placed at the junctions of several one-dimensional
domains within a network. The defect sewing conditions turn out to be quite
restrictive - for example, requiring the number of domains meeting at a
junction to be even - and there is a clear distinction between the behaviour of
conformal and massive integrable models. The ideas are mainly developed within
classical field theory and illustrated using a variety of field theory models
defined on the branches of the network, including both linear and nonlinear
examples.
