A classical concept of a metastable state is a local free energy minimum that may ultimately relax to the stable equilibrium state of a global free energy minimum via an activation process. This process is different from the barrier-free relaxation that spontaneously takes place when an unstable state relaxes. In order to be classified as metastable, the lifetime of a metastable state must be longer than the timescale of observation, which in turn is practically limited by experimental equipment and the patience of the observer. In the classical concept of metastable states there is an assumption that the systems are large enough so that there is no need for consideration of size or other kinetic effects on the system. In general, polymers are much more prone to access the metastable region than small molecules. Polymeric materials, with their various hierarchies of microstructure, may have metastable states present due to small phase size, composition, external fields and other causes. In this review, our focus will be on concepts and experimental observations of metastable states in polymer phase transformations where two ordered structures exist. The phase stability relationships of these structures will be discussed with respect to the phase size, namely, the lamellar thickness. The concepts involved in metastable states and metastability are not only important in the scientific understanding of condensed polymer physics, but are also useful for practical materials development and applications.