Low socioeconomic status (SES) is a well-established risk factor for general and mental health problems. However, there is no widely accepted definition or operationalisation for SES, leading to varied interpretations in research. In a critical review of the child and adolescent mental health literature, we map how SES is defined and measured. We examined 334 relevant papers from 2013 to 2024 and found significant variability in the operationalisation of SES. Our analysis revealed fundamental problems such as the lack of clear definitions, insufficient detail on variables used and limited measures directly reported by adolescents. We discuss issues related to measurement techniques and their impact on reproducibility, policy development and intervention design. Based on our findings, we recommend using SES measures that directly assess the socioeconomic position of children and adolescents. Additionally, we recommend researchers improve transparency and specificity in reporting the measures used and the rationale behind their selection. The wide range of distinct measures used to represent SES, coupled with insufficient reporting, likely hampers our understanding of which underlying factors truly drive observed effects and impedes the establishment of causal relationships. This, in turn, makes the path to effective health interventions more challenging.