Book excerpt: We study the impact of Central Bank intervention on the process of price formation in currency markets. We use a unique dataset of tick-by-tick indicative quotes posted by dealers on Reuters terminals and of intraday sterilized spot interventions and customer transactions executed on behalf of the Swiss National Bank (SNB) on the Swiss Franc/U.S. Dollar exchange rate (CHFUSD) between 1986 and 1998. We find that SNB interventions (but not ex post uninformative customer transactions), although being smaller than typical forex trades, had significant and persistent (albeit asymmetric, depending on their sign) effects on daily currency returns, especially when (relatively) large in magnitude, expected by the market, or inconsistent with existing momentum. The market did not anticipate the occurrence of incoming interventions, unless if chasing the trend. The SNB was much less successful in smoothing fluctuations of the currency, for daily CHFUSD volatility always surged in proximity of interventions, as did average absolute and proportional spreads. Decomposition of estimated absolute spread shocks also reveals that SNB actions induced misinformation among market participants, impacted current market liquidity, increased competition among dealers, and reduced trading immediacy. In many cases, these changes translated into higher transaction costs borne by the population of investors.