In this chapter the authors offer their perspective about modern trading systems, widely known as algorithmic traders (ATs) or high frequency traders (HFTs). At a glance, their presence might be considered unfair and unhealthy: their activity is often perceived as purely speculative, based on the edge that such systems have over other market participants, often less informed and slow. However, ATs and HFTs are a natural consequence of technological progress and they are a relevant and unavoidable component of modern financial ecosystems. Therefore, a holistic and comprehensive assessment of their impact on overall market quality seems useful for a better understanding of their actual role. As in the case of many other phenomena that influence our daily life in a pervasive way, a deeper insight is often more valuable than a moral judgment, as it helps building up adaptive strategies and live with them.