The paper reviews the efforts of the Southeastern Radiological Health Laboratory in low-level counting by liquid scintillation. The author has dealt primarily with (3H) counting but have recognized that low-level counting of other radionuclides may be carried out advantageously by the same technique. All investigations reported here have been carried out using a Beckman liquid scintillation system except where specifically mentioned. There are a number of factors affecting the performance of low-level counting systems, and various attempts have been made to correlate these factors in a single formula. The major factors determining the sensitivity of a low-level counting system are background, efficiency, and sample size. Other factors, such as the requirements of time and skill for sample preparation, and the convenience of operation, are important but have relatively little effect on the sensitivity of the system.