ACR TI-RADS borrows from the American Thyroid Association guidelines and defines clinically important growth as a 20% increase in at least two nodule dimensions and a minimal increase of 2 mm, or a 50% or greater increase in volume. If a nodule enlarges to the point where it exceeds the size threshold for its ACR TI-RADS level, we recommend FNA, if this has not already been performed. Although rapid enlargement is suspicious, growth does not reliably distinguish between benign and malignant nodules. However, nodules that do not grow substantially over the course of 5 years (based on comparison between initial and 5-year sonograms) may be considered benign. Nodules that exhibit an interval increase in ACR TI-RADS level but remain below the size threshold for FNA should be imaged with follow-up US in 1 year. The ACR TI-RADS risk-stratification system allows practitioners to determine whether thyroid nodules require biopsy, follow-up, or no further action based on their US appearance. Success demands close cooperation between radiologists and sonographers, understanding of the nuances and pitfalls of feature assignments, and attention to detail in reporting and making recommendations.