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 $6.4 billion, $3.7 billion, and $3.1 billion, at December 31, 2009, 2008, and 2007. Amounts held in foreign currencies were $2.8 billion, $1.7 billion, and $1.2 billion at December 31, 2009, 2008, and 2007, and were primarily Euros, British Pounds, and Japanese Yen. 

Cash provided by operating activities was $3.3 billion, $1.7 billion, and $1.4 billion in 2009, 2008, and 2007. Our operating cash flows result primarily from cash received from our customers, from sellers, and from non-retail activities such as marketing and promotional agreements, Amazon Web Services, other seller services, 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 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. 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, cash outlays for acquisitions, equity-method investments and intellectual property rights, and purchases of fixed assets, including leasehold improvements and internal-use software and website development costs. Cash provided by (used in) investing activities was $(2.3) billion, $(1.2) billion, and $42 million, in 2009, 2008, and 2007, with the variability caused primarily by purchases, maturities, and sales of marketable securities and other investments, partially offset by decreases in cash paid for acquisitions. Capital expenditures were $373 million, $333 million, and $224 million in 2009, 2008, and 2007, with the sequential increases primarily reflecting additional investments in technology infrastructure, fulfillment- related assets and the development of new features and product offerings on our websites. In 2010 we plan to significantly increase 

our capital expenditures for additional investments in corporate office space and in support of