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Electromagnetic applications have been at the forefront of technological advancements for many years. As demand for these grows, the need for materials possessing unique electromagnetic properties increases. The aim of this research is therefore to investigate the electromagnetic properties of the nanoparticle colloids in the frequency range of 1 MHz to 20 GHz, with focus on the electromagnetic absorption mechanisms at microwave frequencies. A broad range of magnetic and dielectric properties are investigated using novel, as well as conventional, theoretical and experimental techniques. A number of mechanisms are highlighted for tailoring the electromagnetic performance. Results from the CoxNii.x series, with particle sizes ranging from 25 nm to 200 nm, show particle size related dielectric and magnetic properties, which aid the optimisation of resulting properties, in addition to conventional mechanisms, which are also demonstrated in colloidal form. Further reduction in particle size to below 20 nm, leads to single magnetic domain particles, which also exhibit enhanced electromagnetic properties, as demonstrated with broadband magnetic performance achieved for Co ferrofluid with an average particle size of 5 nm. Charged particle colloids, which typically consist of particles with negative surface charge suspended in an aqueous electrolyte solution, have also been investigated. These exhibit interesting dielectric properties, which occur over a range of timescales determined by various aspects of the system. It is shown that the small nanoparticle sizes, ranging from 20 nm to 220 nm, lead to faster charge dynamics than those expected from conventional micron particle sizes, such that the loss extends to microwave frequencies. The results are fitted to various models, which are validated through experiment and used to establish the origin of the results. These are used to further optimise the electromagnetic properties achievable from nanoparticle colloids and assess their potential benefits.