The mechanisms of cell nucleation and growth are investigated in foam injection molding (FIM) using gas-counter pressure (GCP). An in-situ mold visualization technique is employed. The application of GCP suppresses cell nucleation, and prevents the blowing agent from escaping during mold-filling. The inherent structural heterogeneity in the regular FIM can be improved because of the uniform cavity pressure when employing GCP. The cavity pressure profiles show much faster pressure-drop rates using GCP, because the single-phase polymer/gas mixture has a lower compressibility than the two-phase polymer/bubble mixture. Therefore, both the cell nucleation and growth rates are significantly increased through a higher pressure-drop rate on the removal of the GCP. The effect of GCP magnitude on the cell morphology is explored. When the GCP is lower than the solubility pressure, bimodal foaming occurs. As the GCP increases above the solubility pressure, the cell density increases because of the higher pressure-drop rate. (c) 2016 American Institute of Chemical Engineers AIChE J, 62: 4035-4046, 2016