Last week the Organization for Economic Cooperation and Development (OECD) released an analysis of union density - the percentage of workers belonging to a trade union - in OECD member states.
In many cases there is an inexorable and uninterrupted decline over the past 50 years. In the case of the UK there was a steady rise in membership between 1967 (39.7%) and 1978 (51.8%) but a period of sharp decline since has seen union density drop to well under one third of the workforce (28%).
http://www.oecd.org/dataoecd/25/42/39891561.xls
The New York Times has produced an interactive chart where trends in each country can be viewed:
http://economix.blogs.nytimes.com/2009/11/05/trade-unions-around-the-world/
There is extensive academic study of union decline with general agreement that the most significant factors are structural changes in employment and industry as well as macroeconomic and political influences such as institutional/legal framework and Government/employer attitudes.
The extent to which union organisational efforts can impact on density remains contentious and is a key question in union debates about models of organising.