Conventional IVF insemination has been providing satisfactory results in most cases in absence of male-factor infertility. However, low fertilization or complete fertilization failure can happen not only when sperm parameters are impaired but occasionally also when there is no detected male factor. In these occasions, to rescue the IVF treatment cycle, ICSI re-insemination of the failed-fertilized oocytes (after the original conventional insemination) may be attempted. Oocyte aging plays an important role, as because fertilization check is usually performed 16–18 hours after conventional insemination, when failure of fertilization can be detected, however, by that time outcomes after rescue ICSI is mostly suboptimal, if attempted. Alternatively, conventionally inseminated oocytes can be examined much sooner to detect signs of fertilization. It has been demonstrated, if oocytes are submitted to rescue ICSI within 4–8 hours after the initial insemination, then laboratory and clinical outcomes are significantly improved. Since conventional IVF insemination remains to be used frequently around the world, adequately performed rescue ICSI is a viable option to save IVF treatment cycles when unexpected low or total fertilization failure occur.