This chapter focuses on one example of the extra-textual content to the proviso to Article 9(1), namely, the concept of the fundamental rules of natural justice (FRNJ). It chapter looks at how 'law' has been interpreted to include the concept of the FRNJ and how this concept has evolved over time. The chapter analyses the way the courts have interpreted FRNJ in subsequent case law. The chapter identifies four different tracks that the discussion on FRNJ has taken in the case law. The two key cases discussing procedural rights within the concept of FRNJ are Ong Ah Chuan and Haw Tua Tau. However, the courts have been willing to incorporate procedural decision-making standards from the administrative law context into the concept of FRNJ. Recent case law indicates movements in the approach to constitutional interpretation that could signal the evolution of constitutional law to embrace more substantive standards in assessing constitutionality issues.