The evolutionary process of interspecific hybridization in cladocerans is reviewed based on ecological and population genetic data. The evolutionary consequences of hybridization, biogeographic patterns and fitness comparisons are analyzed within the conceptual framework of theories on hybridization. Among species of the D. longispina complex no interpopulational transition zones (hybrid zones) have been detected, but rather patchy distributions of hybrids and parentals have been found. Hybrids occur across broad geographic ranges and can be more abundant than parental species. Due to asexual reproduction (ameiotic parthenogenesis), hybrid breakdown can be avoided, and hybrids can even (temporarily) combine advantageous traits of both parental species. Evolutionary consequences may arise from repeated backcrossing, which in some cases results in introgression and patterns of reticulate evolution. [KEYWORDS: Natural interspecific hybridization; introgression; parthenogenesis; life histories; population genetics; daphnia longispina complex Mitochondrial-dna analysis; life-history variation; mosaic hybrid zone; daphnia-pulex; introgressive hybridization;natural hybridization; population-dynamics; gene flow; notropis-cornutus; sunfish lepomis]