sustainable growth in free cash flow. Free cash flow, a non-GAAP financial measure, was $1.36 billion for 2008, compared to $1.18 billion and $486 million for 2007 and 2006. See “Results of Operations—Non-GAAP Financial Measures” below for a reconciliation of free cash flow to cash provided by operating activities. The increase in free cash flow in 2008 primarily resulted from increased operating income, offset by increased capital expenditures. The increase in free cash flow in 2007 primarily resulted from increased operating income. Operating cash flows and free cash flows can be volatile and are sensitive to many factors, including changes in working capital, and the timing and magnitude of capital expenditures. 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, valuation of cash equivalents and marketable securities, 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 $3.7 billion, $3.1 billion, and $2.0 billion at December 31, 2008, 2007 and 2006. Amounts held in foreign currencies were $1.7 billion, $1.2 billion, and $623 million at December 31, 2008, 2007, and 2006, and were primarily Euros, British Pounds, and Japanese Yen. See Item 8 of Part II, “Financial Statements and Supplementary Data—Note 12—Income Taxes.” 

Cash provided by operating activities was $1.7 billion, $1.4 billion, and $702 million in 2008, 2007, and 2006. Our operating cash flows result primarily from cash received from our customers, from sellers, and from non-retail activities such as through our co-branded credit card agreements, other seller services, and miscellaneous marketing and promotional agreements, offset by cash payments we make for products and services, employee compensation (less amounts capitalized pursuant to SOP 98-1 that are reflected as cash used in investing activities), payment processing and related transaction costs, operating leases, and interest payments on our long-term debt obligations. Cash received from customers, sellers, developers, and other activities generally corresponds to our net sales. Because our customers primarily use credit cards to buy from us, our receivables from customers settle quickly. 

Cash provided by (used in) investing activities corresponds with purchases, sales, and maturities of marketable securities, cash outlays for acquisitions, equity-method investments and intellectual property rights, and purchases of fixed assets, including internal-use software and website development costs. Cash provided by (used in) investing activities was $(1.2) billion, $42 million, and $(333) million in 2008, 2007, and 2006, with the variability caused primarily by purchases, maturities, and sales of marketable securities and other investments, and acquisition activity. Capital expenditures were $333 million, $224 million, and $216 million in 2008, 2007 and 2006, with the sequential increases primarily