In this dissertation, we study various aspects of W algebras and W strings, which are generalisations of the bosonic string theory with higher-spin world-sheet symmetries. We present a simple procedure for constructing the complete cohomology of the two-scalar and multi-scalar W3 strings. We present a procedure for computing gauge-invariant scattering amplitudes for W strings. We use this procedure to calculate many three and four-point functions for the multi-scalar W3 string. We study the spectrum of physical states for W[2,s] strings, based on two-dimensional theories with local spin-2 and spin-s symmetries. We explore the relation of the resulting effective Virasoro string theories to certain minimal models. We also study the structure of general W string theories and discuss the W hierarchical structure that emerges.