Invasive plants have a significant detrimental effect on ecosystems globally, with impacts estimated at millions of dollars per invasive species each year. Biological control has long been used as a management tool for invasive plants, as it is considered a long–term cost–effective control strategy. Surprisingly, the impact of biological agents is rarely quantified. Any form of impact evaluation is generally conducted soon after agent release and establishment; with few studies examining the impact of the agents on the population dynamics of the invader, particularly once the agents have been established for a long time. The aim of the research in my thesis is to evaluate the biological control program of groundsel bush (Baccharis halimifolia L. Asteraceae) in Australia. The groundsel bush biological control agents were released up to 40 years ago and no quantitative assessment of agent impact has ever been conducted, despite the fact that the program has cost about $9.6 million. More specifically, the overall aim of this thesis is to investigate the impact of the released biological control agents on individual plants and populations of groundsel bush. In addition, my thesis aims to examine the impacts of climate as a potential confounding factor of the biological control program. My thesis provides a unique example of biological control evaluation by using a combination of observational damage studies, insect exclusion experiments, and statistical, population and climate modelling to assess, a posteriori, the effectiveness of biological control. This is the first time a long term biological control program has ever been evaluated. To assess the efficacy of the agents, I conducted a large field survey to examine whether the agents were distributed throughout the entire range of groundsel bush and if any biotic or abiotic factors influenced their effectiveness. In addition to this, I assessed the effect of the agents on the growth, survival and fecundity of individual plants under field conditions, and subsequently population growth rate. To do this, I used statistical models of observed effects of biological control agent damage and insect exclusion experiments on plant growth and fecundity to parameterise matrix population models. My results indicate that the groundsel bush biological control agents may be patchy in their effectiveness due to factors such as rainfall and plant size. At their current rate of damage, the groundsel bush biological control agents do not reduce plant growth or fecundity significantly. However, simulation models demonstrated that the agents have the potential to reduce individual plant and population growth when damage is at high levels. A reduction in an invader’s population growth rate, following the introduction of biological control agents, does not necessarily signify that the agents were responsible for the reduction. Factors such as land clearing, chemical and mechanical control, ecosystem health and climate may reduce populations of invasive plant species. With this in mind, I developed a series of climate models to examine how the favourability for growth of groundsel bush may change under different climate scenarios. The climate simulations demonstrated that the distribution and abundance of groundsel bush populations may have contracted in the past 50 years (post–biological control agent release) due to changing rainfall and temperature patterns. The results of the research in my thesis clearly show the need for thorough biological control evaluations, and for detailed data to be collected on the target plant’s demography and population sizes pre- and post-agent release. At a minimum, this should enable biological control practitioners to determine some level of agent impact and demonstrate support for further agent releases or integrative management strategies if necessary. Groundsel bush is a significant invader in Europe where biological control has not yet been carried out. Lessons from the evaluation of the Australian biological control program could be applied to new biological control programs elsewhere such as Europe. Overall, my research findings contribute to a better understanding of how to best evaluate a post-release biological control program, using groundsel bush as a case study. This is the first study to demonstrate an effective set of strategies and quantitative tools to evaluate a biological control program, which can be similarly applied to any biological control program and contributes significantly to an area of biological control which has only recently received significant attention.