This article examines the development of complex onsets (CO) during the L1 acquisition of Spanish (Chilean variety) by nine children (ages 1;6 to 2;8). The participants were interviewed at their homes (in Concepción, Chile) twice a month half a year long. Data were obtained by semi-guided conversation and picture-naming. The analysis of the data was done within Optimality Theory (OT), with a hierarchical configuration of constraints for each stage of CO development. Three stages could be identified. 1st stage: reduction of CO to one position, usually filled by the less sonorous segment; 2nd stage: gliding of the second consonant of CC; and 3rd stage: faithful production of CC. These data provide evidence in favour of certain phonological theories: The first stage favours the Sonority Principle of the syllable. The second stage shows that the skeleton develops earlier than the segments. And the third stage slightly favours laterals over rhotics.