Researchers have found that extra-pair paternity prevails among socially monogamous birds and this phenomenon challenges traditional views of mating system by using molecular technology. Contrary to prior expectations, birds are very rarely sexually monogamous since extra-pair offspring has been found in approximately 90% of species. Even among socially monogamous species, over 11% of offspring are, extra-pair paternity on average. Extra-pair paternity arises when female birds engage in extra-pair copulations with males other than their social mates, frequently resulting in broods with multiple paternity which contain offspring sired by the social male and offspring sired by extra-pair males. A large and solid body of evidence has impressively demonstrated that extra-pair paternity is the rule rather than the exception in birds (especially in passerines), and a number of synthetic contributions have reviewed this field with its various ramifications over the last decade. Extra-pair paternity is widespread in avian species so that the reason for the occurrence of extra-pair paternity and the influence has become the focus researches in the behavioral ecology of avian species. Based on the researches about avian extra-pair paternity over the past decade, this paper reviews the research progress and summarizes two major aspects on causes and effects of extra-pair paternity. Investigating the causes of extramarital paternity includes: 1, we explore the different sexes in the evolution of reproductive strategies from gender perspective. Male reproductive strategies designed to increase their output; Female strategies are proposed to ensure that female fertility hypothesis, the food supply hypothesis, the hypothesis of genetic benefits, etc., but still controversial.2, The genetic benefit hypothesis ("good genes" hypothesis, "genetic compatibility" hypothesis, the hypothesis of genetic diversity) proposes that extra-pair sires provide alleles of superior quality and/or better compatibility compared to the social mate, resulting in offspring of higher reproductive value. One frequently adopted approach to test this idea is to compare the performance of maternal half-siblings in broods with multiple paternity. However, results did not come to a conclusion, some studies failed to reveal any systematic differences in maternal half-sibling performance, even in the same or in closely related species. Furthermore, application of identical experimental protocols (e.g. the phytohemagglutinin test of cellular immunocompetence) in different species with similar extra-pair mating systems produced mixed results. The inconsistencies of results across extra-pair paternity studies may be caused by methodological challenges and/or reflect true and meaningful differences in the respective study systems. 3, Non-genetic benefit-maternal effects influence the evolution of extrapair paternity. Some studies indicated that genetic quality, such as weight, body size, survival and immune responses, etc may be effected by maternal effects.We also summarize environmental factors affecting the occurrence of extra-pair paternity include reproductive synchrony, breeding density, habitat, etc. Different species have different environmental pressures resulting in the difference of extrapair paternity rate. Finally, we make prospects for the future research. Despite nearly a decade of researches to further explain the phenomenon of extra-pair paternity in birds, an certain reason is still unknown. Experimental operation and the improvement of the theory are keys reach a more comprehensive solution to these problems.