The ``Avrami'' equation applies rigorously only to a system which does not change volume on transformation and where the developing regions are spheres. The consequences of the impossibility of achieving these conditions in real systems is discussed. It is shown that excessive approximation combined with the use of weight rather than volume fractions can lead to errors in the exponentnof as much as 0.3. It is emphasized that only at very low conversions can meaningful interpretations be made of the exponent.