This article presents a framework to evaluate HRM in small and medium-sized enterprises (SMEs), using an open systems theoretical perspective. In presenting an open systems perspective the objective is to overcome the limitations of existing theorising in HRM, in particular to facilitate a move away from the ‘small is beautiful’ versus ‘bleak house’ stereotypes evident in much of the literature concerned with HRM in SMEs. The evidence is drawn from six SMEs operating in the Republic of Ireland, using a case study method. The findings show that a complex interplay of external structural factors and internal dynamics shaped HRM in each of the companies. HRM was not the coherent set of practices typically identified in the literature but rather was often informal and emergent. It is argued that the open systems theoretical framework enables a move beyond mere recognition of the complexity and heterogeneity of HRM in SMEs, towards an understanding, accommodation and explanation of particularistic factors.