inventory management and category expansion, the timing of cash receipts and payments, vendor payment terms, and fluctuations in foreign exchange rates. 

Our principal sources of liquidity are cash flows generated from operations and our cash, cash equivalents, and marketable securities balances, which, at fair value, were $8.8 billion, $6.4 billion, and $3.7 billion, at December 31, 2010, 2009, and 2008. Amounts held in foreign currencies were $3.4 billion, $2.8 billion, and $1.7 billion at December 31, 2010, 2009, and 2008, and were primarily Euros, British Pounds, and Japanese Yen. 

Cash provided by operating activities was $3.5 billion, $3.3 billion, and $1.7 billion in 2010, 2009, and 2008. Our operating cash flows result primarily from cash received from our consumer, seller, and enterprise customers, miscellaneous marketing and promotional agreements, and our co-branded credit card agreements, offset by cash payments we make for products and services, employee compensation (less amounts capitalized related to internal use software that are reflected as cash used in investing activities), payment processing and related transaction costs, operating leases, and interest payments on our long-term obligations. Cash received from our consumer, seller, and enterprise customers, and other activities generally corresponds to our net sales. Because consumers primarily use credit cards to buy from us, our receivables from customers settle quickly. Changes to our operating cash flows have historically been driven primarily by changes in operating income and changes to the components of working capital, including changes to receivable and payable days and inventory turns, as well as changes to non- cash items such as excess stock-based compensation and deferred taxes. 

Cash provided by (used in) investing activities corresponds with purchases, sales, and maturities of marketable securities, capital expenditures, including leasehold improvements, internal-use software and website development costs, and cash outlays for acquisitions, investments in other companies and intellectual property rights. Cash provided by (used in) investing activities was $(3.4) billion, $(2.3) billion, and $(1.2) billion in 2010, 2009, and 2008, with the variability caused primarily by increased capital expenditures, increases in cash paid for acquisitions, and purchases, maturities, and sales of marketable securities and other investments. Capital expenditures were $979 million, $373 million, and $333 million in 2010, 2009, and 2008, with the sequential increases primarily reflecting additional investments in support of continued business growth, including investments in technology infrastructure including AWS, additional capacity to support our fulfillment operations, and, in 2010, investments in corporate office space. We expect this trend to continue over time. 

Capital expenditures included $176 million, $146 million, and $128 million for internal-use software and website development during 2010, 2009, and 2008. Stock-based compensation capitalized for internal-use software and website development costs does not affect cash flows. In 2010, 2009, and 2008, we made