For more than half a century, quantum field theory (QFT) has been the most accurate and successful framework to describe the fundamental interactions among elementary particles, albeit with the notable exception of gravity. Nevertheless, QFTs are in general far from being completely understood. This is due to a lack of calculational techniques and tools as well as our limited understanding of the mathematical structures that emerge in them. In the last one and a half decades, tremendous progress has been made in understanding certain aspects of a particular QFT, namely the maximally supersymmetric Yang-Mills theory in four dimensions, termed N = 4 SYM theory, which has risen the hope that this theory could be exactly solvable. In particular, this progress occurred for scattering amplitudes due to the development of on-shell methods and for correlation functions of gauge-invariant local composite operators due to integrability. In this thesis, we address the question to which extend the methods and structures found there can be generalised to other quantities in the same theory and to other theories.Form factors describe the overlap between a gauge-invariant local composite operator on the one hand and an asymptotic on-shell scattering state on the other hand. Thus, they form a bridge between the purely off-shell correlation functions and the purely onshell scattering amplitudes. In the first part of this thesis, we calculate form factors of general, protected as well as non-protected, operators at various loop orders and numbers of external points in N = 4 SYM theory. This is achieved using many of the successful onshell methods that were developed in the context of scattering amplitudes, albeit after some important extensions. In particular, we show how form factors and on-shell methods allow us to obtain the dilatation operator, which yields the spectrum of anomalous dimensions of composite operators and acts as Hamiltonian of the integrable spin chain of the spectral problem. At one-loop level, we calculate the cut-constructible part of the form factor with minimal particle multiplicity for any operator using generalised unitarity and obtain the complete one-loop dilatation operator from it. We demonstrate that on-shell methods and form factors can be used to calculate the dilatation operator also at higher loop orders, using the Konishi operator and the SU(2) sector at two loops as examples. Remarkably, the finite remainder functions of the latter form factors possess universal properties with respect to their transcendentality. Moreover, form factors of non-protected operators share many features of scattering amplitudes in QCD, such as UV divergences and rational terms. At tree level, we show how to construct form factors via extended on-shell diagrams, a Graßmannian integral as well as the integrability-based technique of R operators. Using the latter technique, form factors can be constructed as eigenstates of an integrable transfer matrix, which implies the existence of a tower of conserved charges.Defor...