Portfolio management is the set of activities that allows a firm to select, develop, and commercialize a pipeline of new products aligned with the firm's strategy that will enable it to continue to grow profitably over the long term. To appropriately manage the firm's new product portfolio, decisions must be made about which projects to fund, to what levels, at what point in time. Previous research has investigated portfolio management decisions as individually discrete decisions. Significant streams of research have investigated both project selection and project termination decisions. This research project shows, however, that portfolio decision making may be better understood if it is considered as an integrated system of processes that considers these decisions simultaneously, along with other decisions such as those to continue a project with reduced funding. Using in-depth data from four diverse case studies, we use a grounded theory approach to develop a general model of how firms make new product portfolio decisions. According to the findings from these cases, effective portfolio decision-making processes produce a portfolio mindset, focus effort on the right projects, and allow agile decision making across the portfolio's set of projects. Effective portfolio decision making is the result of the interaction between three types of decision-making processes that managers use in making decisions: evidence-, power-, and opinion-based. Being able to use each of these types of processes to make decisions depends upon having the data inputs that they require. Three domain-based decision input-generating processes (i.e., cross-functional collaboration, practices of critical thinking, and practices of market immersion) are associated with making evidence-based portfolio decisions. In addition, organizational politics produces the inputs that are associated with power-based portfolio decision making, while managerial intuition is associated with opinion-based portfolio decision making. Firm cultural factors, including trust, collective ambition, and leadership style, are associated with how these evidence-, power- and opinion-based processes are combined into an overall portfolio decision making process, and whether the firm's processes are more rational and objectively made, or more politically and intuitively made. The article presents propositions for how the decision-making processes interact in their associations with decision-making effectiveness.