This chapter defines first heuristically and formally the mathematical notion of fractality and explains why introducing it became necessary to study highly irregular geometric sets, in particular those resulting from self-organization processes such as cities. It then reviews the main applications of the notion to the study of urban morphology, as well as the theories attempting to explain its origin in the specific context of cities. Next, it reviews the literature discussing the validity of presenting a single fractal exponent to describe an entire city, together with a brief recall of the discussion around the same issue for the related notion of urban scaling. This naturally leads to the definition of the mathematical notion of multifractality which solves both issues by allowing to combine, for a given urban measure, a multitude of local scaling exponents with a multitude of local fractal exponents into a single spectrum of values.