The use of renewable energy is critical in anticipating the rise in greenhouse gas emissions driven by growing energy consumption, which is the sector that contributes the most to global emissions (73.2%). Hydrogen and ammonia are renewable energy alternatives that can be utilized to minimize reliance on fossil fuels, which are still the world's primary energy sources, and speed the energy transition, particularly in the power generating sector. Fuel-blended hydrogen co-firing in power plants has several benefits, including reduced reliance on a single energy source, increased resilience to supply disruptions, integrated renewable energy to overcome grid intermittency, and improved energy efficiency due to hydrogen's high specific energy and flammability range. Retrofitting gas turbines for hydrogen co-firing provides fuel flexibility and minimizes infrastructure modifications. Ammonia, as an alternative carbon-free energy carrier, contributes to the energy transition through its high thermochemical storage capacity and established infrastructure. However, before large-scale application, investigations are required to assess the effects on flame stability, pollutant emissions, material embrittlement, toxicity, and autoignition. Several experts are evaluating hydrogen and ammonia combustion in gas turbines, with technology progressing rapidly to increase the blending ratio. The main challenge is increasing the co-firing ratio in natural gas-fueled power plants and justifying gas characteristics in combustion systems with co-firing technology. This chapter investigates the combustion behaviour, emissions, and performance of hydrogen and ammonia fuel blends in gas turbines using co-firing technology. It examines the optimal fuel blend ratios for gas-fired power generation. The study reveals that hydrogen and ammonia use in gas turbines offers both challenges and benefits. Increasing hydrogen percentages significantly reduces carbon emissions and enhances gas turbine performance due to hydrogen's high specific energy. However, higher temperatures can lead to pre-ignition or flashback. Therefore, it is recommended to use a moderate fuel blend ratio to avoid flashback zones and ensure that nitrogen oxides and carbon monoxide emissions meet environmental standards for gas turbine engines.