This research assesses private global governance in the form of private sustainability standards. It looks both upstream at factors impacting the shape these standards take and downstream at who gets certified and how standards impact the opportunities for growth for small producers. The thesis addresses these questions in the context of the Kenyan cut-flower sector and uses qualitative and quantitative methods and an agent-based model to find answers. The first paper proposes a way to understand the design of private sustainability standards by retrieving the incentives of different types of standard setters – leading to explanations which defy the standard expectations of greenwashing. The second paper finds that uptake of standards in the flower sector happens depending on the specific value chain a farm is in. Particularly flower farms delivering to supermarkets are certified. This pattern does not hold for standards set to protect the reputation of the sector as a whole. The third and last paper assesses the impact of private sustainability standards on prospects for growth of small growers. It finds that the impact of standards depends on sector characteristics, in particular the capital intensity of the sector, the preferences of certificate demanding buyers to work with large producers and the extent to which standards reduce transaction costs, allowing buyers to work with more suppliers. This research improves our understanding of the shape private sustainability standards take, the role of different value chains in the uptake of standards and context factors mediating the impact of standards on growth prospects for small producers.