  The isoscaling parameter usually denoted by $\alpha$ depends upon both the
symmetry energy coefficient and the isotopic contents of the dissociating
systems. We compute $\alpha$ in theoretical models: first in a simple mean
field model and then in thermodynamic models using both grand canonical and
canonical ensembles. For finite systems the canonical ensemble is much more
appropriate. The model values of $\alpha$ are compared with a much used
standard formula. Next we turn to cases where in experiments, there are
significant deviations from isoscaling. We show that in such cases, although
the grand canonical model fails, the canonical model is capable of explaining
the data.
