This chapter explores cognitive impairments, particularly impairments in memory, concentration, planning, and cognitive flexibility as transdiagnostic features of serious mental illness (SMI). Etiologically, these impairments are intrinsic to SMI, often predating symptom onset and persisting across stages of mental health problems. Epidemiological data indicate that 50–80% of individuals with SMI exhibit impairments in domains such as processing speed, attention, working memory, and executive function, with schizophrenia spectrum conditions showing the greatest severity. Dimensional assessments using widely used neuropsychological performance measures reveal heterogeneous profiles of impairment that correlate broadly but nonspecifically with functional outcomes. There is a need for culturally sensitive performance measures to determine the extent of neurocognitive impairments across cultures. Behavioral strategies emphasize integrating neurocognitive and functional assessments, recognizing that impairments in social cognition and theory of mind further compound difficulties in daily functioning. A dimensional, behaviorally anchored approach to assessment and intervention highlighting the need for operational clarity and targeted supports to enhance adaptive functioning and quality of life in individuals living with SMI.