Recent reports on CaCu 3Ti 4O 12 (CCT) have attributed the anomalously large permittivity to Maxwell-Wagner effects due to the presence of space charge [I. P. Raevski et al. J. Appl. Phys. 93, 4130 (2003)]. In this work, TEM analysis on polycrystalline samples revealed unique microstructural features including dislocation networks on the micrometer scale and twin-like structures on the 10 nm scale. In addition, the dielectric spectra exhibited two relaxation-like features at approximately 100 K and 400 K. The low temperature permittivity was relatively insensitive to processing conditions. However, the high temperature permittivity increased dramatically with increasing oxygen partial pressure during annealing. This suggests that while the presence of charge carriers plays a role in the dielectric response of CaCu 3Ti 4O 12, they are not solely responsible for the anomalously large room temperature permittivity.