Phase transformation toughening (PTT), characterized by a transition from a metastable to a stable crystalline phase and associated with volume expansion, is widely used for toughness improvement of metals and ceramics. The essential prerequisites of this toughening concept are met also for semicrystalline polymers since they are polymorphous and capable to undergo transformation from one crystalline phase to an other under mechanical stresses. The transition is, however, mostly connected to volume contraction, i.e. just the opposite what is being exploited in toughened steels and ceramics. It was argued that an irreversible transformation from a less dense crystalline phase to a more dense one could also be beneficial for toughness upgrading in polymers. This transition is combined with changes in lamellar and molecular level resulting in a "strain-hardened" microstructure, that can be considered as the best possible response of the polymer upon mechanical loading ("adaptive" toughening). The main advantage of the PTT concept is that toughness improvement is achieved without incorporation of foreign materials which is the guarantee of easy recycling.