Reproductive polyphenism, which allows one genotype to produce sexual and asexual morphs, is an extreme case of phenotypic plasticity and is commonly observed in aphids.  Aphids are typical species that switch these reproductive modes, and the pathway orientation is triggered by the environmental conditions (mainly photoperiod and temperature).  The typical annual life of aphids includes a succession of parthenogenetic generations during the spring and summer and a single sexual generation in autumn.  In this review, we describe how the environmental cues orientate the reproductive mode of aphids from photoperiodic perception to endocrine regulation, and how juvenile hormones may act on the target cells (oocytes) to initiate the gametogenesis and embryogenesis in sexual and asexual reproduction.  We also discuss the paradox of sex, especially the advantages of sexual reproduction in aphids.  With the recent development of genomic resources in aphids, many potential genes involved in the reproductive polyphenism will enter the public’s awareness.  In particular, we describe a novel RNAi method in aphids, which may provide a molecular technique for determining the developmental fate and multiple reproductive strategies.