This chapter provides an overview of definiteness marking in North American Norwegian (NAmNo), and compares present-day NAmNo to historical NAmNo and homeland Norwegian. Three types of definiteness marking are discussed: (i) indefinite determiners in singular indefinite phrases; (ii) the definite suffix in unmodified definite phrases; and (iii) double definiteness in modified definite phrases. Review of the available data and studies shows that present-day NAmNo is largely characterized by stability, although some aspects of the grammar are found to be more vulnerable to change. The first main conclusion of this chapter is that the syntax of definiteness is largely stable, while the morpho-phonological realization is less robust. This is in line with other findings in heritage languages. The second conclusion is that while indefinite determiners and definite suffixes are stable in present-day NAmNo, there has been an innovation in double definite phrases, where the prenominal determiner is often omitted. The chapter discusses theoretical implications of these findings, and discusses the factors that have made double definiteness especially vulnerable in NAmNo. At the end of the chapter, some venues for future research are identified.