(en) This thesis centres around the issue of R&D, investment and growth: it is mainly concerned with the rationale which shapes the optimal investment pattern defined by a firm over time in a growing economy. We start considering the economy as a whole in order to evaluate how its actual functioning (interaction between research labs, risk of failure in innovating, patent laws and so on) shapes firms' specific investment policies. More precisely, we focus on the R&D process as it really develops in the market, and examine what is the optimal investment policy for firms involved in research and whether the selected investment policy in turn sustains the growth path of the economy. After analysing how the economic environment may affect firms' investment plans, we propose to identify the decision mechanism, as it develops inside the firm, through which an investment policy is defined over time. Thus, we move to an in-depth analysis of the process through which a profit-maximizing entity selects a specific investment policy when it is required to expand its production plant in order to satisfy a growing economy.