Abstract: Battles between the males and females over reproductive decisions, such as how often to mate and how much to invest now versus later, are widespread and appear to drive much of the reproductive exuberance we see in nature. Much progress has been made over the past few decades in understanding why and how these battles occur. Broadly speaking, in situations where reproducing partners have little shared interest in each other’s reproductive futures, they can sometimes benefit by exploiting, rather than fully cooperating with, their mates. Much work into the details of how these sexual conflicts are manifested has been done in Drosophila fruitflies. This has revealed a fascinating covert battleground between males and females mediated by the actions of seminal fluid proteins passed from males to females during mating along with sperm. These proteins affect the behavior and physiology of the female in dramatic ways, by altering her egg laying, sexual receptivity, feeding and immune genes. Males can sometimes gain by using these proteins to make females invest more in current versus future reproductive episodes, even though this can be harmful to females in the longer-term, e.g. by shortening their lifespan. In this situation, females should evolve resistance to these proteins to avoid this harm. Two ‘stand out’ features of seminal fluid proteins involved in mediating sexual conflict are (a) their functional complexity and (b) their strong interactions with the environment. As well as summarising our current understanding of this field I will explore the origin and maintenance of this complexity and suggest why some male reproductive strategies are ‘resistance proof’ and hence persist, while others aren’t and don’t.