To review the evidence regarding a decrease in worldwide sperm parameters and discuss potential causative factors. The topic of worldwide decline in sperm parameters is contentious; however, recent high-quality studies have demonstrated that there is indeed a decline in sperm parameters. Several retrospective and basic science studies have shown possible links for this decline in sperm parameters such as obesity, diet, and environmental toxins. There exist substantial data to suggest a decline in sperm counts over time. Although causative factors have yet to be fully elucidated, potential causes include, increased rates of obesity, poor diet, and exposure to environmental toxins. How this decline in sperm counts reflects fertility has yet to be determined. As such, further studies are necessary to evaluate whether this decline in sperm count correlates with decreased fecundity and how to identify and mitigate potential causative factors.