<p>A castellated steel beam (CSB) is fabricated by flame cutting of a steel I-beam through its centreline in a zig-zag pattern and rejoining the cut parts to produce a beam with increased overall depth and hexagonal web openings. CSBs are widely used in steel structures due to their beneficial properties, such as reduced weight, increased load-carrying capacity, and a good strength-weight ratio. However, their increased depth can lead to failure from web post-buckling and lateral torsional buckling under load, making stiffeners essential for reinforcement. Stiffeners of mild steel (MS) are more frequently used for reinforcing CSBs. However, in recent years, fibre-reinforced polymers (FRP) have been widely used for strengthening CSBs due to their lightweight property. Further, the use of Carbon Fibre Reinforced Polymer (CFRP) in CSBs has been proven to be a better material to act as a stiffener in enhancing the load-carrying capacity of CSBs. This paper provides a rigorous literature review focusing on the behavior of CSBs with different stiffener materials. The review indicates that while MS stiffeners improve load-carrying capacity and reduce deflection, they also increase the beam’s weight and are susceptible to corrosion. In contrast, CFRP stiffeners offer enhanced load-carrying capacity and reduced deflection while simultaneously decreasing overall weight and eliminating corrosion risks. The study concludes that there is significant scope for investigating the non-linear behavior of CSBs with different shaped openings, both without and with CFRP stiffeners.</p>