Studies on identity in general and mathematical identity in particular have gained much interest over the last decades. However, although measurements have been proven to be potent tools in many scientific fields, a lack of consensus on ontological, epistemological, and methodological issues has complicated measurements of mathematical identities. Specifically, most studies conceptualise mathematical identity as something multidimensional and situated, which obviously complicates measurement, since these aspects violate basic requirements of measurement. However, most concepts that are measured in scientific work are both multidimensional and situated, even in physics. In effect, these concepts are being conceptualised as sufficiently uni-dimensional and invariant for measures to be meaningful. We assert that if the same judgements were to be made regarding mathematical identity, that is, whether identity can be measured with one instrument alone, whether one needs multiple instruments, or whether measurement is meaningless, it would be necessary to know how much of the multidimensionality can be captured by one measure and how situated mathematical identity is. Accordingly, this paper proposes a theoretical perspective on mathematical identity that is consistent with basic requirements of measurement. Moreover, characteristics of students' mathematical identities are presented and the problem of "situatedness" is discussed.