Developmental psychopathology is defined by the investigation of the relationship between development and psychopathology. The transdiagnostic approach signifies a return to defining the specific factors underlying a range of disorders (Nolen-Hoeksema and Watkins, 2011). It seeks to define the relative contribution of different factors to the development of specific disorders and to use the same factors to describe the underlying symptoms in a broader category of disorders (Ehrenreich-May and Chu, 2013). Attention-deficit/hyperactivity disorder (ADHD) is a neurodevelopmental disorder defined by two dimensions: Attention and Hyperactive-Impulsive Behaviour. Defiance and Opposition Disorder (ODD) is characterised by disinhibition and three dimensions: angry/irritable mood, argumentative/defiant behaviour and vindictiveness (APA, 2014). ADHD and ODD exibit high comorbidity and variability in terms of dominant comorbid factors across the lifespan. ADHD that begins in early childhood may be a risk factor for the development of ODD. In addition, Nigg and Barkley (2014) describe emotional dysregulation as a link between ADHD and ODD, as it includes emotional impulsivity, defined as an individual's emotional response to various events that occurs more quickly than expected. Theory of Mind (ToM) can be defined as a component of social cognition within which the attribution of thoughts, feelings, intentions or beliefs and the understanding of their role as drivers of behaviour stands out as significant in guiding social interaction and individual behaviour in social situations (Bernstein et al., 2011). Hutchins et al. (2016) define ToM as a multi-faceted construct that includes joint attention, beliefs, understanding of rules of the game, empathy, intentions, the ability to distinguish what is real and what is not, and the ability to distinguish between the cognitive and physical world. It involves affect recognition, first and second order thought understanding, visual perspective taking and the ability to understand that seeing something leads to understanding something. Wellman and Liu (2004) and Wellman (2018) point out that one should use a complex measure such as the Theory of mind scale of the Test of emotion comprehension (Pons and Harris, 2000) to capture the whole construct of ToM. Children diagnosed with ADHD and/or ODD have impaired social interaction, so Theory of Mind can be considered a transdiagnostic factor in the development of these disorders. Furthermore, there is not much research on preschool children that includes ToM as a transdiagnostic factor for ADHD and ODD, but some research suggests that ToM is important in explaining difficulties with general and emotional adaptability and peer interactions (Heleniak and McLaughlin, 2020; Nader - Grosbois et al., 2013). In this research, an attempt was made to test the transdiagnostic role of theory of mind in explaining some externalised problems in preschool children. All instruments used in this research were used for the first time in Croatia for testing preschool children, and their psychometric properties were verified. The structure of the ADHD and ODD scales suggests a vague clinical picture and the possibility of a unique factor for externalised problems, supporting theories that emphasise ambiguity in differentiating disorders at an early age (Moreno-De-Luca et al., 2013). The scales used to study ToM do not have adequate psychometric properties. These findings are explained by the misconception of ToM as a unidimensional construct and the inability to fit it into a single conceptual framework. Warnell and Redcay (2019) note that children and adults show inconsistencies and partial success on different tasks of the ToM, and highlight the importance of defining ToM as a multidimensional process associated with a range of different abilities. Similarly, Hutchins et al. (2016) define ToM as a multifaceted construct that encompasses a range of constructs such as joint attention and empathy and consists of cognitive and affective components. Furthermore, the findings of this research indicate that the theory of mind does not play a significant transdiagnostic role in explaining ADHD and ODD in preschool children. Finally, the major contribution of this dissertation lies in the conceptualisation of externalised problems in preschool children and the definition of ToM measures.