Although current programming models provide adequate performance, many prove inadequate to support the effective development of efficient Grid applications. Many of the hard issues, such as the dynamic nature of the Grid environment, are left to the programmer. We are developing a programming model that incorporates a familiar, formal computational model and a reflective interface. The programming model, called PAGIS, provides a usable abstract computer in its own right, with an additional interface to both introduce and customize Grid functionality. Using PAGIS, an application programmer constructs applications that are implicitly parallel and distributed transparently. I will discuss the basic components of the PAGIS framework for constructing and executing applications, and the reflective techniques to customize applications in support of adaptive Grid software. This work supports our view that reflective techniques are an invaluable part of the toolkit of a grid application PSE designer.