Due to the ever increasing electromagnetic density in our environments, the ability to manipulate radio signals is more important than ever. Specifically, the rise of IoT (internet of things) and 5G has opened up new avenues across various frequency ranges which highlights the need for techniques to control the bandwidth of structures. This thesis explores the use of metamaterials and Frequency Selective Surfaces (FSSs) to design passive, ultra-thin, flexible, broadband structures with unique electromagnetic properties. The research explores the design, modelling, fabrication, and measurement of several structures with different notchbands, with an emphasis on minimising the profile and controlling the bandwidth using passive techniques. A novel fabrication technique was developed to create small, conductive elements on an ultra-thin, flexible, and optically transparent substrate for use in both single and multilayered structures. Laser engraving was used to selectively remove a conductive layer from a dielectric substrate to create an array of conductive elements of prescribed geometry. This method allows for rapid prototyping of surfaces and incurs low fabrication costs, given the process and inexpensive materials used. A technique for bandwidth control of simple loop type bandstop FSSs is explored with the inclusion of additional elements within the same surface. This resulted in the creation of additional resonances resulting in both multiband and broadband responses. The pattern and orientation of an element array was explored, highlighting its importance on controlling the bandwidth, as well as introducing a polarisation dependant response. Further control, and an increase in the bandwidth of bandstop FSSs, were also explored by cascading additional surfaces to create multi-layered structures. These structures incorporated dual elements and a translation between layers to further improve the bandwidth and polarisation stability, with the ability to shift the resonant frequency. A complex, miniaturized element based on a convoluted square loop element was introduced, and was used in conjunction with the previously explored techniques to create a dual layer structure with dual element translated elements resulting in a -10dB fractional bandwidth (FBW) of 118% along with a frequency reduction of 70%. Ultra-thin and flexible reflection and transmission polarisers operating in the X-band were designed, and applied to structures to significantly increase the bandwidth, whilst minimising the profile of the structure, with the added benefit of being flexible and conformal to curved surfaces. These structures rotate the polarisation of the incident wave through the use of FSS layers and dielectric spacers, where the geometry of the FSS allowed for selection based on the polarisation of the incident wave.