Cross-informant concordance (peers vs. teachers), temporal stability, and reliability associated with sociometrics were systematically examined in a sample of 84 preschoolers (M = 4.5 years). Structural equation modeling analyses revealed that parallel forms of teacher and peer sociometrics measured overlapping and unique aspects of peer popularity. Reliability, a characteristic of a measurement instrument (sociometric assessment), was differentiated from stability, which is associated with the phenomenon or behavior being measured (children's peer popularity). Teacher-measured popularity was highly stable over an 8-week period, while peer-measured popularity showed lower stability. High reliability was found for both teacher and peer sociometrics. Child age and classroom participation rates did not alter the pattern or magnitude of the modeled relationships.