Early indicators of anxiety risk can appear as early as infancy, informing developmental pathways in which individual differences in temperament elevate the likelihood of future anxiety disorders. Clarifying the mechanisms that connect these early biological predispositions to later anxiety offers a foundation for designing targeted early intervention and prevention efforts. In this chapter, we aim to describe the association between fearful temperament and the development of anxiety disorders, highlighting how the interplay between biological and environmental factors shape vulnerability to anxiety from early in life. We describe (a) fearful temperament as a potential marker for vulnerability to anxiety, (b) neural mechanisms underlying fearful temperament and anxiety through detection and regulation processes, (c) internal and external factors that moderate the association between fearful temperament and anxiety, focusing on attentional bias and parental factors to understand distinct etiological process.