Objectives The aim of this study was to analyze the relationship between beam angulation and air kerma in a modern cardiac catheterization laboratory. Background Recent reports have identified the merits of reducing radiation scatter, an important determinant of radiation dose in the catheterization laboratory. Radiation scatter is poorly characterized in the context of catheterization laboratories using modern digital equipment. Understanding the principles of dosimetry may reduce the radiation exposure to patients, providers, and medical staff. Methods Prospectively captured radiation data were extracted from a database of 1,975 diagnostic catheterizations (DCs) and 755 percutaneous coronary interventions (PCIs), which included 138,342 fluoroscopic and 35,440 acquisition (cine) sequences. Fluoroscopy and acquisition modes were categorized into tertiles based on the total air kerma measured at a standard reference point. Radiation maps were modeled according to the relative proportion of exposure in each projection. Results Median air kerma during DCs and PCIs was 677 and 2,188 mGy, respectively. Fluoroscopy contributed to 66.3% of total dose during PCIs compared with 39.7% during DCs (p < 0.001). Fluoroscopy was more sensitive to changes in angulation with a rapid increase in total air kerma on small increases in beam angulation. Complex spatial maps were created to study the impact of angulation and other covariates on total air kerma. Besides beam angulation, body surface area was the strongest predictor of the total air kerma. Conclusions This study uniquely describes radiation dosimetry using contemporary equipment in a real-world setting. Extreme angulations were associated with high air kerma values. Fluoroscopy compared with acquisition was more sensitive to changes in angulation, with relatively larger increases in total air kerma with small increases in steepness of the angulation. © 2014 by The American College of Cardiology Foundation Published by Elsevier Inc.