Despite their ubiquitous nature, there is currently a paucity of theorising and research on the drivers of human resource management (HRM) development within the context of small and medium size enterprises (SMEs). Mainstream HRM models have tended to be biased towards normative prescriptions regarding 'best practice', often drawing insights more applicable to larger organisations with more formalised HRM systems. A principal objective of this PhD thesis is to illustrate the complexities and subtleties of life in small firms: to move beyond those accounts that criticise SME's for their informal agenda and a lack of uptake of best practice HRM to provide a more nuanced understanding of the drivers of HRM development and change in the context of the SME. Through in-depth case studies undertaken in two small firms operating within the aerospace sector, the thesis moves away from prevailing dichotomisations of formal vs. informal or 'small is beautiful' (Schumacher, 1973) vs. 'bleak house' (Sisson, 1993) in an attempt to provide an insight into the lived experiences of SMEs and to bring the voice of the SME and its workers and managers into the story. Moreover, rather than one universally applicable static model, the findings highlight various contingencies linked to growth and firm life-cycle stage that shape HRM development, as well as idiosyncrasies linked to the history and founder management culture. The broader regulatory context of the aerospace sector further illustrates how diverse institutional structures inhabiting the 'regulatory space' (Hancher and Moran, 1989) of organisations are shaping human resource management practices and processes of formalisation. Taken together, there appears to be a complex process of adaption occurring within the context of interacting external and internal contingent forces, which are negotiated within internal micro-politics and agency dynamics between owners and other actors. Such notions of heterogeneity, fluidity and agency are largely underrepresented in mainstream human resource literature and have broader implications for understanding HRM development within the context of SMEs. The thesis represents a broader re-examination of HRM and organisational growth in the SME, providing recognition of the need to bridge the gaps between regulation and governance debates and HRM and organisational debates on SMEs, thus bringing together a variety of perspectives that highlight the conceptual and theoretical limitations of each approach.