The cyclic Mode I delamination growth characteristics of unidirectional carbon/polyetheretherketone (PEEK) composites at ambient and elevated temperatures was studied. At room temperature, crack growth rate was found to increase as a function of cycles, a common characteristic for such materials under load-controlled conditions. In contrast, at elevated temperature 93°C (200°F), early delamination growth was characterized by decreasing growth rate (stable growth). With continued cycling, there was a breakdown of the process zone (craze-plastic zone) leading to rapid crack propagation. Increased temperature and lower rates of deformation tend to enhance ductility in PEEK-based thermoplastic composites through fibril formation resulting from the breakup of crystalline lamellae. This ductility tends to stabilize the crack front, most likely through craze formation ahead of the crack tip, resulting in an anomalous decrease in growth rate with increasing cycles. This behavior is termed as slow crack growth (SCG). The process zone (craze-plastic zone) eventually gives way to crack growth after a critical crack-tip opening displacement is reached. The crack growth process is associated with less ductility, even at elevated temperature 93°C (200°F). When crack growth occurs without any attendant process zone ahead of the crack, as in the case of the specimens tested at room temperature, less ductility is evident. These fracture surfaces display evidence of interlamellar failure. As a result of ductility and the active role of the process zone at elevated temperature, neither the strain energy release rate, G, nor the stress intensity factor, K, were found to be appropriate parameters to characterize the earlier part of delamination growth at elevated temperature. Instead, the process zone (craze-plastic zone) growth at elevated temperature was shown to be best described by a relaxation controlled growth model.