It must look outside for capital - either by borrowing or by issuing stock in the equity markets.
Given the market's ravenous appetite for Internet stocks over the late 90s, Yahoo has understandably financed most of its expansion with equity.
It had its initial public offering in 1996, and since then it has issued stock to pay for its many acquisitions.
It has little long-term debt, which means it doesn't have to worry about interest payments.
Overall, its balance sheet looks very healthy.
In contrast, Amazon.com AMZN, another highly successful Internet company, has borrowed over $2 billion and is highly leveraged in 1999.