This chapter discusses portfolio of securities. A portfolio of securities is a collection of different financial assets owned by an investor, including stocks, bonds, mutual funds, and other investments. The purpose of a portfolio is to diversify investments, balance risk, and help achieve specific financial goals, such as capital growth or income. A detailed analysis of a portfolio of two and three securities is carried out. Markowitz portfolios are considered, as well as Tobin portfolios of n-securities.