Depressive mood, anxiety, delusions, hallucinations and behavioral disturbances have been traditionally recognized as leading symptoms of mental disorders. However, cognitive symptoms went under-recognized or declined. Today there is robust evidence that cognitive dysfunction is present in the majority of mental disorders and is also related to impairments in the functioning of the persons with mental illness. It is proposed that aberrant brain neuronal network connectivity, arising from interplay of genetic, epigenetic, developmental and environmental factors, is responsible for cognitive decline. In schizophrenia, dysfunctions in working memory, attention, processing speed, visual and verbal learning with substantial deficit in reasoning, planning, abstract thinking and problem solving have been extensively documented. Social cognition - the ability to correctly process information and use it to generate appropriate response in situations, is also impaired. The correlation of cognitive impairment with functional outcome and employment, independent living and social functioning has emphasized the need for development of the treatments specific to cognition. It is considered that brain neuroplasticity allows for re-modulating and compensating the impairment process which could give opportunity to improve cognitive functions. Therefore, there is a need for comprehensive clinical assessment and follow-up of cognitive decline in mental illness. Implementation of specific treatment strategies addressing cognitive decline in mental illness, like new drugs, distinct cognitive-behavioural therapy, psychoeducation, social skills training and remediation strategies should be strongly indorsed targeting recovery and reduction of disability due to mental illness.