  The Ising square lattice model with nearest-neighbor (nn) interactions
($J_1$) is one of the few exactly solvable models [1]. Adding next-neareast-
neighbor (nnn) interactions ($J_2$) or a magnetic field (or both) leads to the
non solvability of the model and only some approximate solutions are possible.
In this brief report we will review some results obtained within effective
field theory. We will show that besides second-order transitions there are also
lines of first-order transitions and the coordinates of tricritical points are
calculated.
