This work addresses slums definition according to UN-Habitat, and provides some objectives of impact evaluation of slum upgrading projects. The main objective is to clarify the importance behind the slum upgrading projects (interventions), through over viewing the slum upgrading interventions and their main characteristics then summarize methodology of evaluation studies. This paper gives an overview of counterfactual question, qualitative and quantitative methodologies discusses main strategies to control bias in Quasi-experimental approaches, and provides a summary about common past evaluation challenges and recommendations which can be applied to future work.