Animals, like humans, differ as individuals from one another in the way they consistently express their behaviours across multiple contexts and time. In this thesis, I investigate the personality of sanctuary chimpanzees and rehabilitant orangutans. While leaning on an in-depth review of methodologies available to date, I focus on examining contextual and temporal consistency of personality across multiple distinct daily contexts, using mainly naturalistic observations. By following the animals in their daily activities, I record several naturally occurring behaviours, previously shown to reflect four personality traits: Sociability, boldness, explorativeness, and anxiety. The chimpanzees’ personality assessment was further explored by comparing different methods (i.e., ratings, experimental and naturalistic observations) to understand the contribution of each method for assessing the four personality traits. Firstly, I found evidence of both contextual and temporal consistency of personality-linked behaviours in chimpanzees and orangutans. The findings suggest that chimpanzee and orangutan personality can be reliably measured within ecologically relevant contexts and over time using naturalistic observations. Additionally, the results revealed different patterns of personality consistency across individuals, suggesting that some individuals may be more malleable than others in the expression of their personality-linked behaviours. Finally, I found that the three methods measured similar facets of targeted traits, highlighting the importance of developing methods that can capture the diversity of the expression of a personality trait. Overall, this thesis provides empirical evidence that a naturalistic method can measure personality consistency of both sanctuary chimpanzees and rehabilitant orangutans across distinct daily contexts and over time. By combining a naturalistic approach with different methods of assessment, we can better encompass the complexity of personality, in terms of how individuals express personality, how personality is measured or the factors that may influence its expression.