Background: The Cactaceae is a diverse and conspicuous Neotropical family that has evolved a wide variety of adaptations during co-evolution with their interacting species. Recent research has indicated complex ecological and evolutionary interactions involving cacti and other organisms.Aims: We reviewed four studies involving cacti that have important implications for our understanding of the evolution of life traits and maintenance of cactus diversity. Also, these studies illustrate how the modern theoretical background of the ecology and evolution of species interactions is influencing the research in Cactaceae.Methods: The studies showed here are (1) the evolutionary ecology of a mistletoe-cactus parasitism in central Chile, (2) the effect of an exotic grass on the demography of a threatened cactus in Puerto Rico, (3) the herpetochory in a tropical genus of cacti in Venezuela, and (4) the role of abiotic and biotic factors on the floral morphology in globose cacti species in northern Chile.Results: The parasitic interaction between the mistletoe and a columnar cactus highlights the importance of spine length as a defensive co-evolving trait. Reproductive success in the endemic cactus in Puerto Rico was depressed by the presence of the exotic grass. Lizards actively fed on fruits of a tropical group of cacti, increasing the germination percentages of seeds. Climatic gradients might induce morphological change in the flowers of Mediterranean cactus species.Conclusions: Interactions involving cacti reflect a complex scenario of ecological and evolutionary processes which may account for several patterns of the diversity of Cactaceae. In this sense, to enhance the on-going research, we emphasise the necessity of the development of a cactus interaction database; the assessment of detrimental effects of invasive species on cactus diversity; and the quantification of multi-species interactions.