The global framework for aviation is given by growth expectation (growth rates of 2–5% p.a.) in combination with the challenge to reduce the environmental impact. Especially airlines are under increasing pressure due to ambitious CO2 reduction targets. To reduce fleet emissions, airlines can purchase modern and fuel efficient aircraft or apply retrofits to the existing fleet (e.g., blended winglets). Decisions on these alternatives are part of airline fleet planning where the development of fleet size and composition is determined. Focusing on the transition towards energy-efficient aviation, this paper investigates the influence of emission thresholds and retrofit options on airline fleet planning by making use of an optimization model. Based on real-world data, the model is applied to two major European airlines for a planning horizon between 2016 and 2025. The results indicate that emission thresholds and retrofits can make a significant contribution to achieving short term emission targets. However, the potential is limited due to existing investment budget constraints and the fact that retrofits are only available for short- to medium-haul aircraft. This calls for the development and certification of further retrofit programs as well as the deployment of further measures such as bio-/electrofuels or hybrid electric aircraft.