stock-based compensation expense. Tax benefits relating to excess stock-based compensation deductions are presented in the statement of cash flows as financing cash inflows; accordingly, as such tax benefits decline, a greater amount of cash is classified as operating cash inflow. Operating cash flows and free cash flows can be volatile and are sensitive to many factors, including changes in working capital, the timing and magnitude of capital expenditures, and our federal taxable income. Working capital at any specific point in time is subject to many variables, including seasonality, 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 $9.6 billion, $8.8 billion, and $6.4 billion, at December 31, 2011, 2010, and 2009. Amounts held in foreign currencies were $4.1 billion, $3.4 billion, and $2.8 billion at December 31, 2011, 2010, and 2009, and were primarily Euros, British Pounds, and Japanese Yen. 

Cash provided by operating activities was $3.9 billion, $3.5 billion, and $3.3 billion in 2011, 2010, and 2009. 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 consumers 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 used in investing activities corresponds with capital expenditures, including leasehold improvements, internal-use software and website development costs, cash outlays for acquisitions, investments in other companies and intellectual property rights, and purchases, sales, and maturities of marketable securities. Cash used in investing activities was $(1.9) billion, $(3.4) billion, and $(2.3) billion in 2011, 2010, and 2009, with the variability caused primarily by changes in capital expenditures and changes in cash paid for acquisitions, and purchases, maturities, and sales of marketable securities and other investments. Capital expenditures were $1.8 billion, $979 million, and $373 million in 2011, 2010, and 2009, with the sequential increases primarily reflecting additional investments in