In this chapter we present a fundamental model-theoretic construction due to Roland Fraïssé, which produces countable \(\mathcal {L}\) -structures by amalgamating finite \(\mathcal {L}\) -structures. The resulting amalgams are unique up to isomorphism and possess a rich automorphism group: they are known as Fraïssé’ limits. The first four sections of this chapter provide an introduction to Fraïssé’ limits. After some preliminaries on embeddings, covered in Sect. 12.1, Fraïssé’ limits are defined, proved to exist and to be unique in Sect. 12.2. Section 12.3 describes two very simple examples of Fraïssé’ limit. Section 12.4 proves quantifier elimination for the theory of a Fraïssé’ limit in a finite relational language, from which the limit’s saturation easily follows. The second half of this chapter discusses a fascinating application of Fraïssé limits to measurement theory due to Peter Jephson Cameron. Section 12.5 provides the necessary measurement-theoretic background. Sections 12.6 and 12.7 discuss, respectively, the model-theoretic and group-theoretic results needed to prove the main theorem from Sect. 12.8. This theorem provides information about the possible, abstract scale types supported by the ordered rational numbers: it turns out that there are infinitely many more than can be supported by the ordered reals! Throughout this chapter, \(\mathcal {L}\) is a countable language.