Human error has been identified as the primary contributing cause for up to 80% of the accidents in complex, high risk systems such as aviation, oil and gas, mining and healthcare. Many models have been proposed to analyze these incidents and identify their causes, focusing on the human factor. One such safety model is the Human Factors Analysis and Classification System (HFACS), a comprehensive accident investigation and analysis tool which focuses not only on the act of the individual preceding the accident but on other contributing factors in the system as well. Since its development, HFACS has received substantial research attention; however, the literature on its reliability is limited. This study adds to past research by investigating the overall intra-rater and inter-rater reliability of HFACS in addition to the intra-rater and inter-rater reliability for each tier and category. For this investigation, 125 coders with similar HFACS training coded 95 causal factors extracted from actual incident/accident reports from several sectors. The overall intra-rater reliability was evaluated using percent agreement, Krippendorff"s Alpha, and Cohen"s Kappa, while the inter-rater was analyzed using percent agreement, Krippendorff"s Alpha, and Fleiss" Kappa. Because of analytical limitations, only percent agreement and Krippendorff"s Alpha were used for the intra-rater evaluation at the individual tier and category level and Fleiss" Kappa and Krippendorff"s Alpha, for the corresponding inter-rater evaluation. The overall intra-rater and inter-rater results for the tier level and the individual HFACS tiers achieved acceptable reliability levels with respect to all agreement v ACKNOWLEDGMENTS First, I would like to express my sincerest thanks and gratitude to Allah, who is the source of my success for accomplishing this research. I acknowledge the insightful instruction and guidance of my advisors, Dr. Anand Gramopadhye and Dr. Scott Shappell, who have given me continuous support throughout this research. I also thank my committee members Dr. Kurz and Dr. Sharp, for their valuable suggestions for improving the quality of this work. I am especially grateful to Dr. Julia Sharp; her guidance, constructive feedback, and support significantly contributed to the accomplishment of this research. Special thanks also go to Barbara Ramirez, Director of the Class of 1941 Studio for Student Communication, for her technical help and support in editing this research. Much appreciation goes to my family: my parents, Omar and Mohra; my siblings, Sassia, Abdel-Hakim, Eman, Najmeddien, Wafa, and Housameddien; and my mother in law, Aisha. I am blessed that you are my family and am especially grateful for your prayers, thoughtfulness and emotional support. Special gratitude is also extended to my other family members, aunts, uncles, nieces, nephews, and cousins. Finally, I am very grateful to my husband, Ahmed; I could not have done any of this without you.