Edge effect is the modification of ecological patterns and processes that occur around the edge of two adjacent ecosystems. Depending on their aim, edge effect studies have adopted one of the following methodological approaches: (1) the one-sided approach—which studies ecological patterns and processes from an edge to the interior of just one of the habitats and (2) the two-sided approach—which studies ecological patterns and processes across the whole gradient from the interior of one habitat to the interior of the other habitat, passing through the edge zone. A database containing information on 317 published papers revealed that both methodological approaches were equally used until the end of the 1980s. During the 1990s, the question of how organisms respond to habitat destruction and fragmentation led to an abrupt increase in the number of one-sided studies. Recently, however, two-sided studies have become more frequent. In this review, we put forth theoretical arguments of why the two-sided edge effect approach can produce a broader understanding of the ecological processes associated with edges. We highlight that two-sided edge effect studies must become more experimental and predictive, focusing on the factors controlling edge dynamics. Finally, we point out that two-sided edge effect studies have the potential to create a positive research agenda for the restoration and expansion of endangered ecosystems.