Electromagnetic metasurfaces (MSs) present a challenge for modelling and numerical simulations, as they are inherently "multi-scale". Thus, equivalent models—namely zero-thickness sheets described by surface susceptibilities—are usually employed for numerical simulations. At the same time, space-time varying media have received interest in the past years due to the exciting phenomena they facilitate (e.g. non-reciprocity). This thesis primarily considers the intersection of these two research directions: the modelling of MSs with periodic variation in space and/or time, allowing a Floquet expansion to be used to calculate scattered fields in an efficient way. This thesis also presents susceptibility extractions for some canonical structures, including a ground plane with a dielectric cover layer. Finally, experimental results are shown for MSs which were fabricated, operating in the Ka-band, as a demonstration of the developed Floquet method. These were measured using a near-field (NF) system developed at Carleton University, as a contribution of this thesis.