The power conversion equipment which are used to interface many distributed energy resources (DER) with electricity supply networks are inherently sources of some degree of power quality (PQ) disturbances which must be managed to ensure that the power systems to which they are connected continue to operate in a safe, secure and efficient manner. This chapter focuses on PQ in a DER context. In the first instance, the philosophies used for management of PQ from a customer, equipment and network service provider perspective are detailed. The impacts of DER on each of the most common PQ disturbances (supply voltage magnitude, unbalance, harmonic distortion, voltage fluctuations and flicker) are then evaluated. This evaluation includes a definition for each disturbance, as well as a summary of their impacts on both customers and electricity supply networks. An assessment of the potential impacts of DER on PQ disturbance magnitudes in low-voltage electricity supply networks is then provided. Other than a brief section related to harmonic emissions from larger renewable energy generators (solar farms and wind farms) at the end of the chapter, the focus is on small-scale DER connected to low-voltage networks.