The primary source of income in commercial real estate is rent. Rental rates are driven by a variety of supply and demand factors which make up a separate market for rentable space. As investors consider an acquisition, they must project future movements of this market as it relates to the specific asset. If the space market is expected to yield future increases in rental rates, investors will pay a higher price for the current income stream, pushing the cap rate down. If the space market projects a weak outlook, investors will want to pay less, and cap rates will rise.