Book excerpt: One of the reasons why nanoscience is so fascinating is that the size of the nano-systems is "just about right". They are large enough to have various complex properties, and at the same time they are small enough to be treated (calculated) by numerical methods. I studied the interaction between individual gold nanoparticles (in which plasmons are excited) and quantum dots (in which excitons are excited) experimentally and theoretically at the nanoscale. Coupling optical emitters (quantum dots) to plasmon resonances in metal nanostructures has long been investigated as a means to increase their spontaneous emission rates. This increased rate occurs for weak coupling between the emitter and plasmon; for intermediate coupling, a Fano resonance will be seen; under strong coupling, the system undergoes Rabi splitting into new, hybrid modes. To date, efforts at achieving strong coupling between plasmons and single emitters have mostly been studied in scattering measurements; however, Rabi splitting in the scattering spectrum is difficult to distinguish from the Fano interference, or induced transparency, that occurs at intermediate coupling strengths. Here, we report measurements of scattering and photoluminescence from individual coupled plasmon-emitter systems that consist of a single quantum dot in the gap between a gold nanoparticle and a silver film. Splitting of the modes in photoluminescence is a signature of the strong coupling effect. The measurements unambiguously demonstrated weak, intermediate, and strong coupling at room temperature. In a separate study, we found that the photoluminescence of a quantum dot splits into two modes as a nanometer-sized gold tip approaches the quantum dots on a gold substrate. The splitting in both cases was over 150 meV, which exceeded the plasmon cavity loss (definition of the strong coupling). These studies opened up the possibility of single-photon nonlinearities and other extreme light-matter interactions at the nanoscale at room temperature.