This thesis provides theoretical investigations and developments to endogenous growth models, with the purpose of contributing to growth theory with new mechanisms for endogenously generating positive per-capita long-run growth. Chapter 2 is dedicated to the analysis of the models that constitute the basis of endogenous growth theory. This analytical study is motivated by the need to identify, in the existing growth literature, the mechanisms for generating endogenous sustained positive per-capita growth. In Chapter 3 we develop a new model that integrates R&D and human capital accumulation into a single framework. This growth model has an R&D-based structure although with human capital accumulation as the engine of growth. We build on Romer's [1990] model by introducing two further functions: (1) A specification for the production of new designs that assumes no externalities from the stock of designs into researchers productivity; and (2) A specification for the accumulation of human capital technically similar to that in Lucas [1988]. Our proposed model displays two main results. The first is the elimination of the scale effects prediction. Such a prediction is rejected empirically, hence it is important to eliminate it from growth models. The second result is a new prediction that growth depends negatively on the ratio of final-good-workers to researchers. In Chapters 4 and 5 we develop two growth models with the key aspects of complementarity between capital goods in the production function, and the assumption of internal costly investment in capital. This assumption of internal costly investment in capital is new to the R&D-based growth literature. The two proposed models build on Evans, Honkapohja and Romer's [1998] model, and give microfoundations to the convex adjustment costs in Evans et al. [1998] by introducing an analytically observable internal adjustment cost function due to Hayashi [1982]. The model we introduce in Chapter 4 has a one-sector structure, with the final good, accumulation of capital and the invention of new designs provided by the same technology. In contrast, the model we introduce in Chapter 5 has a two-sector structure, as the final good and capital accumulation are produced with one technology, whereas the invention of new designs is undertaken with another. The main result of our proposed models in Chapters 4 and 5, is that, and in contrast to their inspirational multiple equilibria model by Evans et al. (1998), the combination of the assumptions of complementarities between capital goods and of internal adjustment costs generates a unique equilibrium solution. We then carry out investigations into the possibility of generating multiple equilibria in the two models through the introduction of an analytically observable alternative convex internal investment cost function. Chapter 6 is dedicated to the discussion of some limitations of the existing endogenous growth models and Chapter 7 closes the thesis with Final Remarks.