The Fluid Catalytic Cracking (FCC) process is one of the key units in a modern refinery. Traditionally, its design is primarily aimed for the production of gasoline from heavy oil fractions, but as co-products also diesel blends and valuable gasses (e.g. propene and butenes) are formed in considerable quantities. Environmental legislation, the fact that oil resources will become scarcer and heavier, and the rise of alternative (sustainable) feedstocks call for an evaluation of the feasibility of the FCC process in the future. In this Ph.D. study the feasibility of cracking behaviour of aromatics and (aromatic) sulfur compounds have been studied. In addition, the potential of alternative feedstocks, such as paraffinic waxes produced by Fischer-Tropsch Synthesis (FTS) process and vegetable oils have been evaluated. For this purpose a laboratory-scale microriser reactor has been used with which it is possible to investigate the cracking behaviour of these feedstocks under different process conditions. Catalyst formulation, temperature, catalyst-to-oil (CTO) ratio, and residence time have been varied in order to gain insights in the catalytic cracking mechanisms that take place during Fluid Catalytic Cracking. This enables the optimisation in the processing of different feedstocks.