Weak gravitational lensing is a powerful probe of alternative theories of gravity on cosmological scales. In order to best employ this observation in testing modified gravity, it is crucial to understand the degeneracies at play. I present two approaches towards this goal. First, I discuss a novel expression for the convergence power spectrum under alternative theories of gravity. Obtained by considering small deviations from GR, this expression demonstrates how various physical effects of modified gravity may contribute to the convergence power spectrum. In doing so, it allows us to understand quantitatively the degeneracies which arise between modified gravity parameters in weak lensing observations. Secondly, I present a study of the theoretical uncertainty inherent to the lensing statistic E_G. This statistic has the potential to break degeneracies which may affect the standard weak lensing power spectra. By improving our understanding of the theory behind E_G, we can use it to make more robust statements regarding the viability of alternative theories of gravity.