We first focus on the finite-gap formalism for type IIB strings in AdS_3 x S^1, which allows to encode the semiclassical spectrum of a very large family of string solutions in a Riemann surface, the spectral curve. Then, we show that, in the large angular momentum limit, it separates into two distinct surfaces, allowing the derivation of an explicit expression for the spectrum, which is correspondingly characterised by two separate branches. The latter may be interpreted in terms of two kinds of spikes appearing on the strings: "large" spikes, yielding an infinite contribution to the energy and angular momentum of the string, and "small" spikes, representing finite excitations over the background of the "large" spikes. On the other side of the AdS/CFT correspondence, we consider the sl(2) sector of N=4 super Yang-Mills theory. The corresponding 1-loop spectrum, in the large conformal spin limit, is also encoded in a spectral curve and characterised in terms of the so-called holes. We show that, with the appropriate identifications and with the usual extrapolation from weak to strong 't Hooft coupling described by the cusp anomalous dimension, the large-S spectra of gauge theory and of string theory coincide. Furthermore, we explain how "small" and "large" holes may be identified with "small" and "large" spikes. Finally, we discuss several explicit spiky string solutions in AdS_3 which display the finite-gap spectrum. We compute their spectral curves in the large S limit, finding that they correspond to specific regions of the moduli space of the finite-gap curves. We also explain how "large" spikes may be used in order to extract a discrete system of degrees of freedom from string theory, which can then be matched with the degrees of freedom of the dual gauge theory operators, and how "small" spikes are in fact very similar to the Giant Magnons living in R x S^2.

200 pages, 37 figures, PhD thesis