Quantity and timing of rainfall vary among years in semi-deserts on the inland edges of Mediterranean-climate regions on all continents. One way that plants and animals might cope with the lack of predictability in rainfall is to have many fixed-season breeding attempts over a long lifespan, some of which are likely to succeed. Another approach is to breed sporadically, whenever conditions favour reproduction. In this review we compare breeding phenology of semi-desert organisms with that in other more predictable ecosystems. We test the hypotheses that breeding is unsynchronised within (a) communities and (b) species. We also examine evidence that opportunistic and multiple within-year breeding attempts in plants and animals is characteristic of semi-deserts with episodic rainfall that may occur at any time of the year. We conclude that opportunistic and iteroparous breeding occurs in many unrelated taxa, but that there are life-history, metabolic or dietary constraints to opportunism.