Firm-level product innovation theory today is ambiguous, conflated, fragmented, incomplete, unbalanced and inconsistent. This paper addresses these shortcomings and distinguishes between the overall process of product innovation (new product development) and its outputs of new-to-the firm, internally-sourced product invention, externally-sourced product adoption, and product commercialization. Because product innovation theory involves multiple domains (decision-making, processes and outputs), a multitheoretical approach is taken that employs four theoretical building blocks of related theories that explain firm-level product innovation: evolution (evolution, path-creation, path-dependence theories), intelligence (organizational learning, rational search, knowledge management theories), decision-making (limited rationality, contingency, real options theories) and resources/capabilities (resource-based view, dynamic capabilities theories). This paper formulates a firm-level conceptual framework where strategic decision-making and resources develop absorptive capacity needed for product invention/adoption. Firm product invention/adoption, together with its developed absorptive capacity and complementary assets, achieve product commercialization necessary for firm success.