Nanoparticles of many varieties are increasingly studied for use in the physical, chemical, and biological sciences. Metallic nanoparticles exhibit morphology-dependent localised surface plasmon resonances (LSPR), which couple to propagating light, and manifest as a resonant particle polarisability at the LSPR frequency. These resonances can be harnessed for a variety of applications. Many of these applications require characterisation of NP properties, such as their optical response, summarised by the ab- sorption and scattering cross sections. Quantitative measurement of individual NPs is technically difficult, and ensemble measurement techniques, such as absorption spectroscopy, are frequently employed. However, individual NP properties can vary significantly, within the ensemble. In this work, we present a novel, and easy to implement, wide-field extinction microscopy technique, capable of analysing hundreds of nanoparticles simultaneously. Using this technique, we are able to characterise individual gold nanoparticles down to 5 nm diameter, and collate the data to produce ensemble statistics. Furthermore, we developed a program for the rapid analysis of the acquired image, enabling implementation by others in a cost-effective and efficient manner. Using the wide-field extinction technique, we have studied several sizes of gold, platinum, silver, and diamond nanoparticles. We used gold nanoparticles to pro- vide a proof of concept, and found good agreement with the literature. We also present an experimental investigation towards an in-vitro plasmon ruler. Coupled metallic NPs exhibit a LSPR, which is dependent on interparticle distance. The four-wave mixing technique we employ is phase-sensitive, allowing measurement of the shift of the res- onance frequency of gold NPs. To provide proof-of-principle of the plasmon ruler, we correlatively studied gold nanoparticle dimers, with transmission electron microscopy, and four-wave mixing microscopy. In this way, we obtained a direct measure of the interparticle distance, and could relate it to the measured phase shift in four-wave mixing.