  We present, as a very general method, an effective field theory to analyze
models defined over small-world networks. Even if the exactness of the method
is limited to the paramagnetic regions and to some special limits, it gives the
exact critical behavior and the exact critical surfaces and percolation
thresholds, and provide a clear and immediate (also in terms of calculation)
insight of the physics. The underlying structure of the non random part of the
model, i.e., the set of spins staying in a given lattice L_0 of dimension d_0
and interacting through a fixed coupling J_0, is exactly taken into account.
When J_0\geq 0, the small-world effect gives rise to the known fact that a
second order phase transition takes place, independently of the dimension d_0
and of the added random connectivity c. However, when J_0<0, a completely
different scenario emerges where, besides a spin glass transition, multiple
first- and second-order phase transitions may take place.
