L-Carnitine (LC) and acetyl-L-carnitine (ALC) are highly concentrated in the epididymis and play a crucial role in sperm metabolism and maturation. They are related to sperm motility and have antioxidant properties. The objective of this review is to summarize the multiple roles played by LC and ALC in male reproduction, and to highlight their limitations as well as their benefits in the treatment of male infertility. A variety of studies support the conclusion that LC and/or ALC at total daily amounts of at least 3 g per day can significantly improve both sperm concentration and total sperm counts among men with astheno- or oligoasthenozoospermia. Although many clinical trials have demonstrated the beneficial effects of LC and ALC in selected cases of male infertility, the majority of these studies suffer from a lack of placebo-controlled, double blind design, making it difficult to reach a definite conclusion. Additional, well-designed studies are necessary to further validate the use of carnitines in the treatment of patients with male infertility, specifically in men with poor semen quality.