Investors have historically demanded a return premium for taking on the risk of illiquidity both in terms of characteristic and systematic liquidity risk. Recent research have presented results suggesting that the liquidity premium is diminishing. The increasing popularity of passive investments such as Exchange Traded Funds (ETFs) have been proposed as a driving force for the declining trend. Despite the popularity of ETFs, there is limited research how they impact the financial markets. The purpose of this thesis is to investigate how the liquidity premium has developed in the United States between 1997 and 2016 and to explore if developments in the liquidity premium can be linked to the capital inflow to the United States ETF market. The...