We present a device for specifying and reasoning about syntax for datatypes, programming languages, and logic calculi. More precisely, we consider a general notion of `signature' for specifying syntactic constructions. Our signatures subsume classical algebraic signatures (i.e., signatures for languages with variable binding, such as the pure lambda calculus) and extend to much more general examples. In the spirit of Initial Semantics, we define the `syntax generated by a signature' to be the initial object---if it exists---in a suitable category of models. Our notions of signature and syntax are suited for compositionality and provide, beyond the desired algebra of terms, a well-behaved substitution and the associated inductive/recursive principles. Our signatures are `general' in the sense that the existence of syntax is not automatically guaranteed. In this work, we identify a large class of signatures wich do generate a syntax. This paper builds upon ideas from a previous attempt by Hirschowitz-Maggesi (FICS 2012), which, in turn, was directly inspired by some earlier work of Ghani-Uustalu and Matthes-Uustalu. The main results presented in the paper are computer-checked within the UniMath system.