Symmetries are omnipresent in physics. From classical mechanics, via solid state physics to particle physics, symmetries provide a means for classification and often lead to deep physical insight. In this thesis, we study symmetries in plasmonics using Transformation optics. We show how Transformation optics can be used to reveal, study and make us of symmetries in practical calculations, by studying a range of plasmonic systems. First, we show that an ellipse and spheroids possess a `hidden' rotational symmetry that becomes apparent when transforming them to a rotationally symmetric structure. Next, we investigate plasmonic gratings and show that a whole class of plasmonic gratings (and other periodic structures) can be related to a translationally invariant slab, thereby inheriting all the slabs spectral properties. In studying the plasmonic grating, we extend the Transformation optics approach to treat periodic systems with extent larger than the wavelength in one direction. Finally, we use Transformation optics to study electron energy loss spectroscopy and cathodoluminescence problems in plasmonics, by mapping the plasmonic nanoparticles under investigation to more symmetric ones. Thus, again using the symmetry of the transformed structures to derive analytical solutions to the problem at hand.