The overall aim of this thesis is to improve the knowledge about how policy instruments can contribute to effective and efficient reductions of greenhouse gas emissions in the freight and maritime transport sectors. Paper I addresses this aim by analysing how policy evaluations contribute with information about whether policy instruments in the freight transport sector have been successful in achieving their targets and how to improve or correct already implemented ones. A meta-evaluation of ex-post climate policy evaluations is carried out, and by analysing the outcomes and quality of the evaluations, the study investigates whether estimated effects of policy instruments can be compared between evaluations and if the results are appropriate to use for evidence-based decision making. The study shows that there is a lack of systematic climate policy evaluation which hinders reliable conclusions about the effects of policy instruments. Consequently, there is a need for more systematic monitoring and evaluation of implemented policy instruments and the study suggests that evidence-based decision making can be improved by adjusting current policy evaluation guidelines and by introducing an evaluation obligation. Paper II addresses the overall aim by developing a modelling tool, referred to as the Swedish Energy Transition of Shipping (SETS) model, that can be used for policy scenario analyses of shipowners’ investment decisions in the Swedish maritime transport sector over the time period 2020-2045. The main contribution of the developed SETS model is that it can take into account data for individual ships and their operational patterns when estimating the impact of potential policy instruments. Hence, the model can contribute to an improved understanding of how proposed policy instruments can affect future greenhouse gas emission reductions in the maritime transport sector.