Formative evaluation, a rigorous assessment process to identify potential and actual influences on the implementation process, is a necessary first step prior to launching any implementation effort. Without formative evaluation, intervention studies may fail to translate into meaningful patient care or public health outcomes or across different contexts. Formative evaluation usually consists of qualitative methods, but may involve quantitative or mixed methods. A unique aspect of formative evaluation is that data are shared with the implementation team during the study in order to adapt and improve the process of implementation during the course of the study or improvement activity. In implementation science, and specifically within formative evaluation, it is imperative that a theory or conceptual model or framework guide the selection of the various individual, organizational or contextual factors to be assessed. Data from these theory-based constructs can translate into the development and specification of implementation strategies to support the uptake of the intervention. In this article, we describe different types of formative evaluations (developmental, implementation-focused, progress-focused, and interpretive), and then present a formative evaluation case study from a real-world implementation study within several academic pain clinics, guided by the Theory of Diffusion of Innovation.