Colloidal semiconductor quantum dots are novel materials whose electronic and optical properties can be greatly enhanced from the bulk semiconductor properties by quantum confinement. A brief introduction to colloidal quantum dots and the effect of size on its electronic and optical properties will be given. CdSe/ZnS, PbS, and PbSe quantum dots will be described in more detail with regards to the research done in this dissertation. CdSe/ZnS quantum dots have luminescence that can be tuned in the visible, making them particularly suited for fluorescent labels in biological applications. In particular, their potential use as voltage sensors to detect cell membrane potentials and sphingosine phosphorylation will be discussed. PbS and PbSe quantum dots emit in the near infrared, wavelengths that are important to telecommunications. Their small third-order nonlinearity and relatively long (microsecond) lifetimes will be discussed in terms of dielectric screening. Fluoresence resonant energy transfer between PbS quantum dots will also be discussed.