Crop-growth simulation models have become important tools for researchers and growers for the purpose of assisting management and improving production. The CROPGRO-Tomato model described in this chapter is one of a suite of models within the modelling package Decision Support System for Agrotechnology Transfer (DSSAT), which is widely used and accepted by agricultural research communities (Jones et al., 2003). This model is mechanistic and process oriented (processes of carbon, water and N balance), and it simulates daily progress towards flowering and fruit set as well as daily growth of leaves, stems, roots and fruits over time until maturity or final harvest (Scholberg et al., 1997; Boote et al., 1998; Boote et al., 2012). This chapter describes the model, its sensitivity to climatic and management factors, what it is capable of predicting and how it can be used in various applications of tomato (Solanum lycopersicum Mill.) cultivation in order to address the challenges confronting industry and researchers.