The problem of counting the number of self-avoiding polygons on a square
grid, - therbytheirperimeterortheirenclosedarea, is aproblemthatis
soeasytostate that, at ?rst sight, it seems surprising that it hasn't
been solved. It is however perhaps the simplest member of a large class
of such problems that have resisted all attempts at their exact
solution. These are all problems that are easy to state and look as if
they should be solvable. They include percolation, in its various forms,
the Ising model of ferromagnetism, polyomino enumeration, Potts models
and many others. These models are of intrinsic interest to
mathematicians and mathematical physicists, but can also be applied to
many other areas, including economics, the social sciences, the
biological sciences and even to traf?c models. It is the widespread
applicab- ity of these models to interesting phenomena that makes them
so deserving of our attention. Here however we restrict our attention to
the mathematical aspects. Here we are concerned with collecting together
most of what is known about polygons, and the closely related problems
of polyominoes. We describe what is known, taking care to distinguish
between what has been proved, and what is c- tainlytrue, but has
notbeenproved. Theearlierchaptersfocusonwhatis knownand on why the
problems have not been solved, culminating in a proof of unsolvability,
in a certain sense. The next chapters describe a range of numerical and
theoretical methods and tools for extracting as much information about
the problem as possible, in some cases permittingexactconjecturesto be
mad