The rising global need for green carbon neutral fuels has driven a resurgence of intense research into photoelectrochemical cells. This talk presents an overview of the role that semiconductor modeling can play in understanding and engineering the operation of photoelectrochemical cells. Focusing on the photo-anode, we begin by exploring how the operation of semiconductor/oxide based photocatalytic devices performing water splitting can be understood in a similar framework to solid-state Schottky diodes. From this it is then demonstrated how key properties such as the photovoltage, the photocurrent and carrier trapping can be modeled. Overall, we will discuss the importance of sound semiconductor band diagrams (often referred to as “Kroemer’s Lemma”) to interpreting the experimental operation of photoelectrochemical systems. The aim of this methodology is to accelerate the development of practical photoelectrochemical systems through closer coupling between modeling and experimentation.