In the theory of capital structure, internal financing or self-financing is using its profits or assets of a company or organization as a source of capital to fund a new project or investment. Internal sources of finance contrast with external sources of finance. The main difference between the two is that internal financing refers to the business generating funds from activities and assets that already exist in the company whereas external financing requires the involvement of a third party. Internal financing is generally thought to be less expensive for the firm than external financing because the firm does not have to incur transaction costs to obtain it, nor does it have to pay the taxes associated with paying dividends. Many economists debate whether the availability of internal financing is an important determinant of firm investment or not. A related controversy is whether the fact that internal financing is empirically correlated with investment implies firms are credit constrained and therefore depend on internal financing for investment. Studies show that the availability of funds within a company is a major driver for investment decisions. However, the success and growth of a company is almost entirely dependant on the financial management and the use of internal financing does not explicitly mean success or growth for the firm. The financial manager can use a range of sources including but not limited to retained earnings, the sale of assets, and the reduction and control of working capital to drive expansion and better utilise funds. The availability of internal finance does not have a massive effect on firm growth.

The specific source of internal financing used by a financial manager depends on the industry the firm operates in, the goals of the firm and the restrictions (financial or physical) that are placed on the firm. The sources of internal finance mentioned above can be used in conjunction with one another or individually. The mix of methods that the financial manager would choose also depends on several factors including the goals of the firm, their restrictions and their industry. For example, a retail firm specialising in consumer goods would generally not have as many assets as a car manufacturer. Therefore, the two firms would differ in that the retail firm would rely more on the reduction and control of working capital and retained earnings whereas the car manufacturer would generate more funds through the sale of assets (i.e., plant and equipment).

A big downfall of internal financing revolves around the financial manager and their motives. Financial managers who control large internal sources of finance are more likely to seek investment opportunities that generate lower returns than shareholders can generate for themselves for the purpose of firm growth. Alternatively, managers who source funds externally are monitored closely by the financial market and therefore are inclined to act in the interest of shareholders.

Retained earnings is the most common source of internal financing for a company. Retained earnings are the profits of a company that are not distributed to shareholders in the form of dividends, but rather are reinvested to fund new projects or ventures. Because retained earnings are reinvested rather than distributed in dividends, the company must insure that the investments they make, or the projects they fund using the earnings, yield a rate of return that is equivalent to or higher than the rate of return that investors can generate by reinvesting those dividends that they could have received, all while maintaining the same level of risk. By failing to do so the financial manager may face adverse effects and risk losing shareholders which would lead to a decrease in company value.

The reinvestment of earnings helps current shareholders in that it allows them to maintain the value of their shares. By sourcing the funds internally, a company would not need to issue new shares to raise capital through an IPO thus preventing the dilution of current shareholders' share values. Because retained earnings are funds that are already flowing through the company, the firm does not need to wait to receive those funds, meaning they are readily available for use. It is important to note that companies can do more than just reinvest their earnings or pay them out as dividends. When companies choose to pay out dividends, they only use between 50% and 70% of their earnings, the rest may be utilised elsewhere.

Commonly, most firms rely heavily on internal financing, and retained earnings remains the most prominent form of financing for a firm. Shareholders in a firm are generally happy for retained earnings to be reinvested into the business as long as the projects that the funds are invested in produce a positive NPV. The reason for this is that any projects that are invested in which produce a positive NPV will subsequently increase a shareholders wealth. If internal funds in the form of retained earnings are not enough to cover the cost of an investment then the company faces a financial deficit. In order to overcome this deficit the company would need to cut back on paying dividends in order to increase their retained earnings or alternatively source their funds externally. Studies show that financial managers rely heavily on internal financing because they tend to avoid external financing based on irrational or self-serving fears. For example, by issuing new shares to raise capital the financial manager will be subject to the scrutiny of the financial market and may face awkward questions from potential investors.

Sale of assets refers to a company selling some or all of its assets in exchange for financial or physical gain. These assets can be tangible (physical), intangible (financial), or a combination of both. The sale of assets is an essential aspect of internal financing and one of the more common sources of financing for a company. The assets which a company can sell are not limited. It is down to the financial manager to strategise and decide what assets are to be sold, physical or financial, and the decision is based on either company growth or downsizing.

The sale of assets, through a theoretical perspective, is viewed as a way to increase asset efficiency or raise capital. By increasing asset efficiency, which is done through asset reallocation, companies can take advantage of economic changes and increase their value. This is different to the sale of assets which serves the purpose of generating capital. Both are valid approaches in which a company can initiate growth.