Organisms can often adapt surprisingly quickly to evolutionary challenges, such 
as the application of pesticides or antibiotics, suggesting an abundant supply 
of adaptive genetic variation. In these situations, adaptation should commonly 
produce 'soft' selective sweeps, where multiple adaptive alleles sweep through 
the population at the same time, either because the alleles were already present 
as standing genetic variation or arose independently by recurrent de novo 
mutations. Most well-known examples of rapid molecular adaptation indeed show 
signatures of such soft selective sweeps. Here, we review the current 
understanding of the mechanisms that produce soft sweeps and the approaches used 
for their identification in population genomic data. We argue that soft sweeps 
might be the dominant mode of adaptation in many species.