Parallel processing can provide the huge computational resources that are required to solve todays grand challenges, at a fraction of the cost of developing sequential machines of equal power. However, even with such attractive benefits the parallel software industry is still very small compared to its sequential counterpart. This has been attributed to the lack of an accepted parallel model of computation, therefore leading to software which is architecture dependent with unpredictable performance. The Bulk Synchronous Parallel (BSP) model provides a solution to these problems and can be compared to the Von Neumann model of sequential computation. In this thesis we investigate the issues involved in providing a modular programming environment based on the BSP model. Using our results we present Opal, a BSP programming language that has been designed for parallel programming-in-the-large. While other BSP languages and libraries have been developed, none of them provide support for libraries of parallel algorithms. A library mechanism must be introduced into BSP without destroying the existing cost model. We examine such issues and show that the active library mechanism of Opal leads to algorithms which still have predictable performance. If algorithms are to retain acceptable levels of performance across a range of machines then they must be able to adapt to the architecture that they are executing on. Such adaptive algorithms require support from the programming language, an issue that has been addressed in Opal. To demonstrate the Opal language and its modular features we present a number of example algorithms. Using an Opal compiler that has been developed we show that we can accurately predict the performance of these algorithms. The thesis concludes that by using Opal it is possible to program the BSP model in a modular fashion that follows good software engineering principles. This enables large scale parallel software to be developed that is architecture independent, has predictable performance and is adaptive to the target architecture.