This thesis aimed to identify the clinical, personality, neuropsychological and neurobiological characteristics associated with conduct disorder (CD) problems in a large cohort of community-recruited adolescents. Differences in these characteristics between males and females, and between those with high versus average ‘callous-unemotional’ (CU) traits, were examined. The development of a proxy measure of CU is reported. This was used to explore whether high CU can be used to identify a sub-group of CD youths who differ from their peers in terms of clinical, personality and behavioural phenotypes in this community-recruited sample. Differences in emotional reactivity were investigated using an fMRI paradigm comparing neural responses to angry face stimuli. ‘Cool’ (cognitive) executive function was investigated using an fMRI task assessing motor response inhibition to a visually presented ‘stop-signal’ and a neuropsychological task assessing working memory. The third fMRI study explored ‘hot’ (motivation/affect) executive function using a paradigm investigating reward sensitivity to monetary values at different stages of reward processing (anticipation and outcome) and a neuropsychological task that investigated risk taking through a gambling paradigm. Between-group differences were investigated as a function of CD problems, gender and CU traits. In terms of clinical measures males and females with CD were similar, although considering both CU and gender, in addition to CD, revealed differences in temperament. Emotional reactivity in the amygdala differed as a function of CD status in males, and CU traits differentiated a group of females who show heightened reactivity to negative stimuli. In terms of executive function there were differences as a function of CD and gender for both hot and cool executive function at the neurobiological and behavioural level. The findings are discussed in relation to current theories underlying the development of CD.