Implementation science has evolved over more than 20 years and established a set of theories, models, and frameworks that can be used to guide the implementation process, identify implementation-influencing factors, and evaluate implementation effectiveness. Guided by theories, the implementation research paradigm has been developed to emphasize a problem-oriented approach that seeks to solve problems, identify barriers to implementing evidence-based practices, and selectively employ implementation strategies to facilitate their adoption, implementation, and maintenance. This paradigm also gives implementation research distinctive characteristics in terms of research design, including extensive use of theories, models, and frameworks, alignment with real-world and contextual settings, use of mixed research designs, use of rapid research methods, and optimization of interventions for the context in which they are to be implemented. As the second part of Past and Present of Implementation Science, this paper provides a brief introduction to the theories, paradigms, and characteristics of implementation science.