Interspecific Reproductive Barriers (IRBs) preserve species identity by preventing interspecific hybridization, an essential facet of the biological species concept. Wild tomato species (Solanum sect. lycopersicum) are useful for studying interspecific reproductive barriers. Within the tomato clade there are 13 closely related species possessing diverse mating systems and complex IRBs. IRBs can be divided into two types: those occurring before mating (premating barriers) and those operating after mating (postmating barriers). Premating barriers include a variety of floral morphological characters correlated with a diversity of mating systems. Postmating barriers can be subdivided into prezygotic, those acting after mating but before fertilization, and postzygotic, those acting after fertilization. In the tomato clade, regulation of pollen tube growth in pistils constitutes postmating prezygotic barriers that are known to be important for preventing hybridization. Unilateral incongruity/incompatibility (UI), which prevents hybridization in one direction of an interspecific cross by inhibiting pollen tube growth in the pistil, is common in the tomato clade. Postzygotic barriers are also important as genetic isolating mechanisms resulting in failure of fruit or viable seed production in cases where prezygotic barriers are absent. In this study, I first examined the hypothesis of positive correlation between pollen grain size and style length among nine species in the tomato clade, because differences between species in pollen size and style length have been proposed to be a potentially important isolating mechanism between species, since larger pollen grains (containing more stored nutrients) may be needed to traverse longer styles. However, I found no correlation between pollen grain size and style length in the tomato clade, and therefore did not find this to be a likely isolating mechanism among the species in this study. Second, I examined UI barriers between species of domesticated tomato (self-compatible, SC) and three wild red-fruited SC species as pollen donors onto pistils of eight green-fruited species. Pistils of (self-incompatible) SI green-fruited species rejected pollen from all SC red-fruited species. However, pollen rejection and/or pollen tube growth of the three wild SC red-fruited species varied in pistils of green fruited SC species and SC populations of SI species. Finally, three types of IRBs including stigma exsertion, UI, and postzygotic barriers were investigated in 10 sympatric pairs of wild species. In these sympatric pairs, prezygotic and postzygotic barriers were found to prevent interspecific hybridization. This research will help elucidate the nature of reproductive barriers in wild populations. Studies of IRBs in tomato, a major food crop, also have potential for understanding reproductive barriers as they pertain to agronomic improvement.