Serious mental illness (SMI) is a highly debilitating condition with detrimental consequences if left untreated. Accessible and effective treatments exist, highlighting the importance of identifying people with SMI efficiently to allocate finite healthcare resources to those most in need. The brief unidimensional 10-item Kessler-10 (K10) and 6-item Kessler-6 (K6) scales were developed to meet this need for efficient screening measures to detect SMI. These K10/K6 scales measure behavioral, cognitive, emotional, and physiological aspects of nonspecific psychological distress based on the diagnostic criteria for major depressive episode and generalized anxiety disorder. The original scales had a 30-day recall period, but this can be modified for specific epidemiological and clinical purposes. Epidemiological surveys across various countries consistently show that the unidimensional K10/K6 scales have high concordance with blinded clinician-rated diagnoses of SMI and, at times, any mood or anxiety disorder. These studies consistently show that the K10/K6 scales have strong construct validity and good retest reliability. The K10/K6 have been used in 30 countries worldwide and translated into at least 25 languages. This chapter presents an overview of the development process of the K10/K6 scales, a review of the psychometric properties of the scales as tools for screening for SMI, depression, and anxiety disorders, language translations, cultural considerations, scoring recommendations, limitations, and future directions.