When Sir Nicholas Serota took over as head of the Tate Gallery 20 years ago, Damien Hirst and Tracey Emin were still at college, the Turner Prize was in its infancy, and the Tate was still called the Tate. Many of the changes in the art world since 1988 – the championing of Brit Art, the rise of conceptualism and the dramatic expansion of the Tate itself – can be laid at Sir Nicholas's door. So it is little wonder that he has been reappointed by the Tate's trustees. One can quibble with the secrecy in which they made thedecision – they knew a month ago but kept quiet, and they have notsaid if there were other candidates. Nevertheless, the decision issurely the correct one.

The extent of the Serota influence on art in Britain worries some, who fear that one man with a personal passion for a cutting-edge contemporary art can distort the national picture. But a look at the range of what Sir Nicholas has done disproves this. In establishing Tate Modern, he has given Britain a national gallery of modern art that has housed stunning exhibitions of figurative painting (by Edward Hopper, for example) that are far removed from avant-garde conceptualism. He has also created there a venue that has become one of the capital's key destinations.

But his triumph with Tate Modern can obscure the expansion of Tate Britain, the original Tate Gallery, into a home for British art, the fundraising that gave it new spaces, and the triumph of Tate's own expansion into Liverpool and St Ives. Certainly, it's a big empire for one man, even with highly competent lieutenants. But, Sir Nicholas' great achievement is not just in what he has done with the buildings and the collections. It is what he has achieved in the nation's cultural outlook. We now engage with contemporary art in a way that we did not in 1988. We talk about it; we go to look at it; we are inspired or appalled by it. Sir Nicholas has instigated this national debate on perhaps the most challenging of art forms.

Sir Nicholas's longevity is remarkable at the top level of arts management. None of the other national arts institutions has been in the same hands for anything like such a long period. It is a tribute to Sir Nicholas that he goes on and on. And it is a fitting paradox that in contemporary art his very willingness to take risks makes him the safest pair of hands around.