Inflation reasons beyond Friedman I think everyone know this Friedman's quote nowadays:


  Inflation is always and everywhere a monetary phenomenon in the sense
  that it is and can be produced only by a more rapid increase in the
  quantity of money than in output


However, things are not that simple. The US had emitted lots of money and has very little inflation. In some cases there's also a spiral effect. Some heterodox economists talk about lack of supply of goods too. And emission still applies only to the mid &amp; long term.

To sum up, the questions would be:


Can businesses be an important cause of inflation, or is it a pure monetary effect?
In not-so-developed countries, there might not be much competition.  Can that generate inflation?


Edited. Original question: Can anyone help me understand when that quote is true and when other factors should be considered too?
The US had emitted lots of money and has very little inflation.


Are we talking about quantitative easing here?  It depends how broadly you are willing to define inflation.  Quantitative easing has caused asset price increases, just not consumer price inflation.  Look at stock prices, which are about 50% overvalued according to some well regarded investors (e.g. Warren Buffett).  

A common complaint about quantitative easing is that it is overly focused on investments and not focused enough on people (Wall Street over Main Street).  It has been successful at propping up stock prices to the point that they have returned to their 2008 levels, when they were regarded as being in a bubble.  Quantitative easing is heavily focused on investment assets.  It doesn't seem to be getting money into the hands of consumers.  Household incomes are still low.  

In short, I don't believe that the current US recession has disproved Friedman's statement.  The Federal Reserve has eschewed its traditional open market operations in favor of quantitative easing, which has affected investments without increasing the money supply available to consumers.  A better question might be why quantitative easing is widely regarded as expanding the money supply when it doesn't seem to actually do that--at least not the kind of money supply whose expansion causes consumer price inflation.  

Note:  I'm sure that there is theoretical work disagreeing with Friedman's statement.  I haven't tried to cover that.  I was caught up in the questionable assertion that the US money supply has expanded without causing inflation.  I suspect that this is too new for a consensus to have arisen. This is my thoughts on one possible explanation.