SPSM is a part of the low-cost SPDR Portfolio ETF lineup, a collection of core-exposure funds that track S&P indexes. The fund specifically tracks the S&P SmallCap 600 Index. The S&P's index committee chooses at least 600 securities to represent the US small-cap space, not necessarily the 600 smallest by market cap, which can lead to some omissions of single names. The Committee also considers sector balance by comparing the weight of each GICS sector in the index to its weight in the relevant market cap range of the S&P Total Market Index. SPSM securities are added and removed on an as-needed basis and are rebalanced quarterly. The fund may also invest in futures contracts. Before November 16, 2017, the fund tracked the Russell 2000 Index and traded under the ticker TWOK. Prior to January 24, 2020, the fund tracked the SSGA Small Cap Index. On June 9, 2023, the ETF acquired the assets and liabilities of the SPDR S&P 600 Small Cap ETF (ticker: SLY), which had $1.7 billion in assets.