A characteristic of today’s equity markets is the fraction of trading that occurs in the dark (i.e., outside of regular and visible order books). This study extends investigation of the relationship between transactions that occurred in visible stock markets and those that occurred in dark stock markets. In particular, the study evaluates the quantitative impact of dark trading on the lit London stock market between January 2001 and December 2013. We find that dark trading has a substantial effect on both prices and liquidity in the lit market, counseling for regulation that protects ordinary investors participating in lit stock markets.