Cross-linguistically, numerals differ from other linguistic expressions in various aspects of their grammatical behavior and their acquisition. What is so special about them? I will show that a closer look at the status of numbers and numerals not only gives an answer to this question, but can also shed some light onto the syntax-semantics interface. Taking into account philosophical approaches from the foundations of mathematics, I will set forth a definition of number as a function that can be fulfilled by certain sequences. This will lead us (i) to dispense with abstract entities “numbers“ and (ii) to regard numeral sequences as sets that can function as numbers. I will show that this OCCAMiam view captures the peculiar features of numeral sequences as a reflex of their “number function”. On the other hand, the integration of number words into complex syntactic structures leads to a morpho-syntactic behavior of cardinals, ordinals and numerals in “#”-constructions that comes close to that of different word classes, depending on parallels in their semantic-conceptual structure.