This chapter explores the multifaceted nature of investment within the EU legal order, emphasising its intersection with various areas of law, including national, EU, public international and private international law. It highlights the limited explicit references to investment in the EU Treaties, but nonetheless notes its implicitness in a whole range of areas of EU law. Against this backdrop, the chapter considers the broader implications of investment-related case law of the past decade. It then analyses the tension between the EU’s internal market freedoms and the autonomy of the EU legal order, as well as the impact of recent judgments on intra-EU bilateral investment treaties (BITs) and the Energy Charter Treaty (ECT). It also examines the evolving landscape of EU secondary law on investment, including those related to investment screening and foreign subsidies. Before setting out the trajectory of the book and all the chapters that follow, the chapter notes that investment is deeply embedded in the EU legal order, and its relevance is now influencing the constitutional, internal market, and external relations aspects of the EU.