We report on a qualitative study exploring health social workers' understanding of complexity in relation to inpatients in subacute wards at three sites across a large health network in Melbourne, Australia. Findings indicate that social workers' understanding of complexity refers to five interrelated themes: multiple competing demands; uncertainty; patient and family characteristics; pending breakdown; systems challenges. Social workers with less practice experience report that complex clients present more challenges than do social workers with greater experience. Implications of these findings for advancing practitioners' capacities in working with complex patients are discussed, as are the limitations of the study.