Alliance formation is often described as a mechanism used by firms to increase voluntary knowledge transfers. Access to external knowledge has been increasingly recognized as a main source of a firm's innovativeness. A phenomenon that has recently emerged is alliance portfolio complexity. In line with recent studies this article develops a measure of portfolio complexity in technology partnerships in terms of diversity of elements of the alliance portfolio with which a firm must interact. The analysis considers an alliance portfolio that includes different partnership types (competitor, customer, supplier, and university and research center). So far factors that determine portfolio complexity and its impact on technological performance of firms have remained largely unexplored. This article examines firms' decisions to form alliance portfolios of foreign and domestic partners by two groups of firms: innovators (firms that are successful in introducing new products to the market), and imitators (firms that are successful at introducing products which are not new to the market). This study also assesses a nonlinear impact of the portfolio complexity measure on firms' innovative performance. The empirical models are estimated using data on more than 1800 firms from two consecutive Community Innovation Surveys conducted in 1998 and 2000 in the Netherlands. The results suggest that alliance portfolios of innovators are broader in terms of the different types of alliance partners as compared to those of imitators. This finding underlines the importance of establishing a “radar function” of links to various different partners in accessing novel information. Specifically, the results indicate that foremost innovators have a strong propensity to form portfolios consisting of international alliances. This underlines the importance of this type of partnership in the face of the growing internationalization of R&D and global technology sourcing. Being an innovator or imitator also increases the propensity to form a portfolio of domestic alliances, relative to non-innovators; but this propensity is not stronger for innovators. Innovators appear to derive benefit from both intensive (exploitative) and broad (explorative) use of external information sources. The former type of sourcing is more important for innovators, while the latter is more important for imitators. Finally, alliance complexity is found to have an inverse U-shape relationship to innovative performance. On the one hand, complexity facilitates learning and innovativeness; on the other hand, each organization has a certain management capacity to deal with complexity which sets limits on the amount of alliance portfolio complexity that can be managed within the firm. This clearly suggests that firms face a certain cognitive limit in terms of the degree of complexity they can handle. Despite the noted advantages of an increasing level of alliance portfolio complexity firms will at a certain stage reach a specific inflection point after which marginal costs of managing complexity are higher than the expected benefits from this increased complexity.