Sexual selection has long been known as the force underlying the evolution of male ornaments and armaments that males use to gain access to mates. However, what is less understood is what happens when females mate with multiple males, causing sperm of different males to compete for fertilization. Selection for sperm that are both more competitive and better able to overcome the challenges of the female reproductive tract has brought about tremendous variation in sperm size and shape. I will discuss recent comparative work on the causes and consequences of variation in sperm form and function, including the evolution of the longest sperm ever measured. I will further discuss how males trade off the allocation of their limited resources between producing copious high-quality sperm and the costly ornaments and armaments to gain mating opportunities in the first place.