Various molecules which are similar to the natural nucleobases exist in nature or have been synthetically developed. In this chapter we review work on the photophysical properties of several modified nucleobases, focusing particularly on how these properties differ from those of the natural nucleobases. We discuss studies that give physical insight into how the molecular structure can be related to photophysical properties with many of these studies being theoretical. One useful photophysical property is the ability to fluoresce with high quantum yields. Natural bases practically do not fluoresce, so being able to design molecules that fluoresce is a goal of practical importance. Many of the modified nucleobases discussed in this review are fluorescent analogues, analogues that have very different fluorescent properties from the natural bases. The studies reviewed here may provide ways to design other analogues with a set of desired properties.