Background: Effort estimation is one of the cornerstones in project management with the purpose of creating efficient planning and the ability to keep budgets. Despite the extensive research done within this area, one of the biggest and most complex problems in project management within software development is still considered to be the estimation process. Objectives: The main objectives of this thesis were threefold: i) firstly to define the characteristics for a large project, ii) secondly to identify factors causing inaccurate effort estimates and iii) lastly to understand how the identified factors impact the effort estimation process, all of this within the context of large-scale agile software development and from the perspective of a project team.Methods: To fulfill the purpose of this thesis, an exploratory case study was executed. The data collection consisted of archival research, questionnaire, and interviews. The data analysis was partly conducted using the statistical software toolStata.Results: The definition of a large project is from a project team’s perspective based on high complexity and a large scope of requirements. The following identified factors were identified to affect the estimation process in large projects: deficient requirements, changes in scope, complexity, impact in multiple areas, coordination, and required expertise, and the findings indicate that these are affecting estimation accuracy negatively. Conclusions: The conclusion of this study is that besides the identified factors affecting the estimation process there are many different aspects that can directly or indirectly contribute to inaccurate effort estimates, categorized as requirements, complexity, coordination, input and estimation process, management, and usage of estimates.