The Introduction to Aviation Policies: Studies of Unintended Effects and Consequences critically reexamines the role of academic research in evidence-based policymaking (EBPM). Traditionally, EBPM has focused on estimating the intended effects of policies through rigorous causal inference methods such as randomized controlled trials (RCTs). However, this chapter emphasizes the equal importance of identifying and understanding unintended policy effects. The influence of EBPM is expanding across diverse fields, including transportation, but evidence is frequently underutilized or misused in actual policymaking. This gap stems from multiple factors, including political pressure, institutional constraints, the replication crisis in academia, and the political infeasibility of implementing evidence-based policies. This chapter examines two key perspectives that critique the misuse and underuse of evidence: the normative perspective, which contends that evidence should guide policy, and the realist perspective, which sees policy as shaping both the production and selection of evidence. It then proposes a third approach: the co-production of knowledge. Within this framework, the chapter emphasizes that while causal inference remains a central challenge, the identification of unintended effects and consequences is equally essential. Policy is inherently an imperfect contract and cannot predict all future contingencies. Therefore, analyzing its unintended effects (often latent and easily overlooked) can reveal hidden issues that deserve policy attention. The book presents empirical studies on aviation policies, including airport slot trading, capacity constraints, environmental regulations, and passenger protection. These studies demonstrate how identifying unintended effects can deepen our understanding of complex policy issues and complement EBPM.