In this chapter we explore deterministic finite-state transducers. Obviously, it only makes sense to ask for determinism if we restrict attention to transducers with a functional input-output behaviour. In this chapter we focus on transducers that are deterministic on the input tape (called sequential or subsquential transducers). We shall see that only a proper subset of all regular string functions can be represented by this kind of device and we describe a decision procedure for testing whether a functional transducer can be determinized. Further we present a subsequential transducer minimization procedure based on the Myhill–Nerode relation for string functions.