Abstract Single-system designs (SSDs) are a family of user-friendly empirical procedures that can be used to help professionals to monitor and evaluate the effectiveness of the services they provide to clients and to guide practice. SSDs can be used to evaluate interventions based on any theory or approach. Repeated measurement of the target(s) of intervention is an intrinsic and key element of SSDs. Dozens of SSDs exist, and each has its own strengths and limitations. The most basic and most widely used design is the A-B design. Data from SSDs are analyzed visually, using simple, descriptive or inferential statistics, or using criteria for practical or clinical significance.