Since the development of the first semi-mechanistic oil palm model, OPSIM, the development of new oil palm models has increased in frequency, aiming to take into account aspects of oil palm physiology and the physical processes and causal relationships between the environment and the crop. This chapter describes the development of a new oil palm growth and yield model called PySawit. The chapter discusses the evaluation of PySawit’s accuracy when its predictions were compared with several measured parameters of growth and yield in oil palm aged 1–19 years. The chapter includes a full explanation of the methodology of the model and discussion of its results.