The Fund seeks to meet its investment objective generally by investing in component securities of the Bloomberg Barclays December 2021 Maturity Corporate Index (the “Underlying Index”). The Fund may also invest in other exchange-traded funds (“ETFs”), U.S. government securities, short-term paper, cash and cash equivalents, including shares of money market funds advised by BFA or its affiliates. The Fund is a term fund that will terminate on or about December 15, 2021, at which time it will distribute its remaining net assets to shareholders pursuant to a plan of liquidation. The Fund does not seek to return any predetermined amount at maturity or in periodic distributions. The Underlying Index is composed of U.S. dollar-denominated, taxable, investment-grade (as determined by Bloomberg Index Services Limited (the “Index Provider” or “Bloomberg”)) corporate bonds scheduled to mature after December 31, 2020 and before December 16, 2021. As of October 31, 2020, a significant portion of the Underlying Index is represented by securities of companies in the financials industry or sector. The components of the Underlying Index are likely to change over time. The Underlying Index includes U.S. dollar-denominated, investment-grade securities publicly issued by U.S. and non-U.S. corporate issuers that have $300 million or more of outstanding face value at the time of inclusion. The non-U.S. corporate issuers included in the Underlying Index consist primarily of corporate bonds issued by companies domiciled in developed countries. The Fund will invest in non-U.S. issuers to the extent necessary for it to track the Underlying Index. Each bond must be registered with the SEC, have been exempt from registration at issuance, or have been offered pursuant to Rule 144A under the Securities Act of 1933, as amended (the “1933 Act”), with registration rights. Further, the securities in the Underlying Index must be denominated in U.S. dollars and have a fixed-rate, although they can carry a coupon that steps-up or changes according to a predetermined schedule. In addition, to be included in the Underlying Index, securities that are rated by all three of the rating agencies named below must be rated “investment-grade” by at least two of the agencies, which is defined as Baa3 or higher by Moody’s Investors Service, Inc. or BBB- or higher by S&P Global Ratings or Fitch Ratings, Inc. When ratings from only two of the three rating agencies are available, the lower rating is used to determine eligibility. Securities with a rating from only one of the three ratings agencies must be rated investment-grade in order to be included in the Underlying Index. The Underlying Index is constructed with the following methodology. A parent index, the Bloomberg Barclays U.S. Corporate Index, representing U.S. dollar-denominated, taxable, investment-grade corporate bonds, is stripped of securities maturing outside of the maturity range defined above. During the final two years of the Underlying Index, bonds that had been screened out of the parent index due to being within one year of maturity will be added back into the Underlying Index until such bonds reach maturity. Securities are then market-cap weighted within the Underlying Index, with a 3% cap on any one issuer, and a pro rata distribution of any excess weight across the remaining issuers in the Underlying Index. The securities in the Underlying Index are updated on the last calendar day of each month until six months prior to maturity. The last rebalance date will be on June 30, 2021. During this final six month period, the Underlying Index will no longer be updated or rebalanced, except to remove securities which are downgraded to below investment-grade per the eligibility criteria described above. Additionally, during this period, existing bond weights will be allowed to float based on changes in market value. When a bond that is included in the Underlying Index matures, its maturity value will be represented in the Underlying Index by cash throughout the remaining life of the Underlying Index. As the Fund approaches its termination date, its holdings of money market or similar funds may increase, causing the Fund to incur the fees and expenses of these funds. By December 15, 2021, the Underlying Index value will be represented almost entirely by cash as no securities will remain in the Underlying Index. The Fund is a series of the iShares iBonds® fixed maturity series of bond ETFs sponsored by BlackRock, Inc. (“BlackRock”). The iBonds® fixed maturity series do not invest in U.S. savings bonds or other U.S. government bonds (except to the extent the funds hold cash equivalent instruments consistent with their investment objectives) and are not designed to provide protection against inflation. BFA uses a “passive” or indexing approach to try to achieve the Fund’s investment objective. Unlike many investment companies, the Fund does not try to “beat” the index it tracks and does not seek temporary defensive positions when markets decline or appear overvalued. Indexing may eliminate the chance that the Fund will substantially outperform the Underlying Index but also may reduce some of the risks of active management, such as poor security selection. Indexing seeks to achieve lower costs and better after-tax performance by aiming to keep portfolio turnover low in comparison to actively managed investment companies. BFA uses a representative sampling indexing strategy to manage the Fund. “Representative sampling” is an indexing strategy that involves investing in a representative sample of securities that collectively has an investment profile similar to that of an applicable underlying index. The securities selected are expected to have, in the aggregate, investment characteristics (based on factors such as market value and industry weightings), fundamental characteristics (such as return variability, duration, maturity, credit ratings and yield) and liquidity measures similar to those of an applicable underlying index. The Fund may or may not hold all of the securities in the Underlying Index. The Fund generally will invest at least 90% of its assets in the component securities of the Underlying Index, except during the last months of the Fund’s operations, as described below, and may invest up to 10% of its assets in certain futures, options and swap contracts, cash and cash equivalents, including shares of money market funds advised by BFA or its affiliates (“BlackRock Cash Funds”), as well as in securities not included in the Underlying Index, but which BFA believes will help the Fund track the Underlying Index. From time to time when conditions warrant, however, the Fund may invest at least 80% of its assets in the component securities of the Underlying Index and may invest up to 20% of its assets in certain futures, options and swap contracts, cash and cash equivalents, including shares of BlackRock Cash Funds, as well as in securities not included in the Underlying Index, but which BFA believes will help the Fund track the Underlying Index. The Fund seeks to track the investment results of the Underlying Index before fees and expenses of the Fund. In the last months of operation, as the bonds held by the Fund mature, the proceeds will not be reinvested by the Fund in bonds but instead will be held in cash and cash equivalents. By December 15, 2021, the Underlying Index is expected to consist almost entirely of cash earned in this manner. On or around this date, the Fund will wind up and terminate, and its net assets will be distributed to then-current shareholders pursuant to a plan of liquidation. The Fund may lend securities representing up to one-third of the value of the Fund's total assets (including the value of any collateral received). The Underlying Index is sponsored by Bloomberg, which is independent of the Fund and BFA. The Index Provider determines the composition and relative weightings of the securities in the Underlying Index and publishes information regarding the market value of the Underlying Index. Industry Concentration Policy. The Fund will concentrate its investments (i.e., hold 25% or more of its total assets) in a particular industry or group of industries to approximately the same extent that the Underlying Index isconcentrated. For purposes of this limitation, securities of the U.S. government (including its agencies and instrumentalities), repurchase agreements collateralized by U.S. government securities, and securities of state or municipal governments and their political subdivisions are not considered to be issued by members of any industry.