Looking back over the last few years, there appear to be three significant strands to growth in union membership.
One is organising employers through partnership deals. Another is playing the role of a profession. The third is militancy.
The unions which best exemplify these strands are retail union Usdaw, the NASUWT and NUT teaching unions and the RMT transport union.
Usdaw is now the fourth biggest union in Britain. It has grown from just under 300,000 members at the end of the 1990s to just under 400,000 members last year.
In the same period the NUT has grown from just under 200,000 members to just over 300,000 while its sister union NASUWT has almost doubled its membership to just under 300,000.
Meanwhile RMT grew from 60,000 members in 1997 to 80,000 in 2009 before losing members and then recovering to 77,000 last year.
Before examining these unions it's worth noting something about the labour markets they operate in.
Generally speaking, they have either been expanding or have been relatively stable in recent years - though the creation of part-time employment may mean that the total of full-time equivalent jobs is not as great as may seem the case. This is particularly relevant for Usdaw.
In other words the vast majority of the membership of these unions is not to be found in sectors like manufacturing, which have been hammered in employment levels.
It remains to be seen what the impact of the government's cuts will be on these unions' memberships.
The majority of Usdaw's membership is found among a very small number of very large private-sector employers, the supermarkets. It is able to recruit in large numbers for two reasons.
First, the union is given the status of being legitimate and favoured by the employer, allowing such things as being able to speak to new workers as they take their induction and training courses at the start of their employment.
Second, Usdaw spends vast amounts of money training staff and shop stewards to be recruiters and retainers. Indeed, it has its own organising academy.
This level of expenditure is essential give the scale of staff turnover in retail as a result of both staff moving around and the temporal nature of job contracts.
What Usdaw then does with its membership is a key question. It is well known to be on the right of the union movement and not one which, like RMT, is keen to use the potential industrial muscle of its members. In this sense, its members punch well below their collective weight.
Yet the struggle of Robbie Segal, a member of the Militant/Socialist Party, did show that there was a sizeable audience for opposing partnership with the employer - she won 40 per cent of the vote when she stood for the union presidency in 2008 and served four three-year terms on the union's executive. More recently, some 150 drivers employed by Wincanton on a Sainsbury's contract left Usdaw to join Unite after being unable to get the union to act on their will.
The two teaching unions, which are now working more closely together than ever before, also employ an organising approach to recruitment and retention and have again made serious investments in this strategy.
They have been at the more progressive end of the spectrum in the battle against the erosion of public-sector pensions.
But alongside all this, what they have done successfully is develop their united voice so that they can legitimately claim to be the "voice of the profession."
The way they have done this is to be able to credibly and convincingly position themselves as not just the lead representatives on matters of pay and conditions for teachers, but also as the spokespeople for the profession in terms of standards, ethics and quality.
In other words, they are able to say publicly that government cutbacks to the teaching profession will not only detrimentally affect their employment position but also the education of children.
As producers of education, they have been able to pose as the defenders of the users of education. So far they have not become professional bodies like the British Medical Association or Law Society, which regulate entry to their respective professions and conduct within them.
In the case of RMT the election of Bob Crow to its general secretaryship in 2002 has very much been the talisman around which the union has harnessed the collective power of its members to improve their pay and conditions.
As a union whose members are able to have a significant and immediate impact upon rail services when they withdraw their strategically placed labour, RMT has epitomised the "fighting back" approach of militancy.
In all of this it has benefited from a very strong form of occupational and union identity.
The three different examples, Usdaw, NUT and NASUWT and RMT, do not just highlight that there are different roads to membership growth, but that the differing labour markets do have significant implications for how this growth can be achieved.
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