The principal aim of the present study was to obtain a deeper understanding than hitherto of the concurrent correlates and prospective predictors of loneliness and poor peer acceptance, both falling under the umbrella term low social inclusion. Problematic and socially competent behaviours were investigated as possible predictors of low social inclusion in grade 6, as defined by self-rated loneliness and degree of peer non-acceptance, respectively. In grade 6 808 children participated whereof 323 were followed longitudinally from first grade. Loneliness in grade 6 was distinctively associated with high levels of internalizing problems, concurrently as well as prospectively. Peer acceptance, on the other hand, emerged as a complex, multifaceted aspect, with concurrent, independent predictions from both externalizing and internalizing problem behaviours as well as social competence, although prospective analyses found early externalizing problems to be the strongest predictor of peer acceptance. Moderating effects of social competence were sparse, although peer nominated social competence buffered peer acceptance for children with high levels of aggression in the concurrent analyses and social competence boosted peer acceptance for children with low problem levels in the predictive analyses. Social competence did not appear to buffer the negative impact of early problem behaviours on peer relations. (Contains 4 tables.)