Cast poly(ethylene terephthalate) films were biaxially drawn and subsequently heat-set. All films were drawn 3.5×, at fixed width and at 90°C, in the machine direction (MD). and subsequently drawn to various degrees in the transverse direction (TD). The structure of the films, before and after heat treatment, was investigated using refractive index and density measurement, wide- and small-angle x-ray scattering, and transmission electron microscopy (TEM). The TEM study was performed using intrinsic phase-contrast imaging of thin foils microtomed at an angle to the MD. The MD-drawn material exhibits crystallization in the form of thin fibrillar crytals with chain axes along the MD. TD drawing disrupts this structure and ultimately produces a second crystal population with chain axes along the TD. Subsequent heat treatment produces a fibrillar-to-lamellar transition and a subsequent increase in the degree of crystallinity. Throughout these processes (100) is parallel to the film plane, while (010) lies normal to the film plane.