1 Introduction  
2 Why Explainable AI is Needed?  
3 From Explainability to Interpretability  
4 Systematic Review Planning and Execution  
5 Discussion  
5.1 General Challenges and Research Directions in XAI  
5.2 Challenges and Research Directions of XAI in the Design Phase  
5.3 Challenges and Research Directions of XAI in the Development Phase  
5.4 Challenges and Research Directions of XAI in the Deployment Phase  
6 Conclusions