Introduction: Widely used def index for the assessment of caries around the world, is facing criticism on various limitations. WHO, ICDAS II and puff/PUFA indices had their own merits and demerits. Furthermore, the caries assessment indices documented in the literature does not cover the total caries spectrum. That necessitated the development of a comprehensive but realistic caries assessment index called the __ampersandsignldquo;Caries Assessment Spectrum and Treatment Needs (CAST) index.__ampersandsignrdquo; Objective: To compare CAST and def indices in measuring dental caries among 3-6-year-old school children in and around Bhimavaram town, Andhra Pradesh, India. Methods and Material: A single examiner conducted a descriptive cross-sectional survey among 898, 3-6-year-old school children at randomly selected schools. Caries experience was recorded in a structured proforma using CAST and def indices. Descriptive statistical analysis was done. Inter and intragroup comparison were made by Kruskal Wallis ANOVA and Mann Whitney U tests respectively. Correlation between def and CAST indices was explored by Spearman__ampersandsignrsquo;s rank correlation coefficient. Results: Caries prevalence with CAST index (45.7%) was marginally higher compared with def index (44.7%). No significant difference was found between mean deft scores measured with CAST and def index (p=0.87). The high statistically significant difference in mean def scores with def index and the def component of CAST index for 3, 4, 5 and 6-year age groups was observed (p__ampersandsignlt;0.001). A strong correlation in measuring dental caries (p__ampersandsignlt;0.001)and a similar percentage of agreement for application (98.5%) was observed between both indices. Conclusion: No significant difference was found in measuring caries experience indicating the similarity between CAST and def indices in quantifying dental caries. CAST index provides more detailed information on caries prevalence, experience and severity compared with def index.