Traditionally philatelists collected stamps according to the country of issue, sub-divided into reigns or political periods. A straightforward collection of Grenada or St Vincent, for example, would progress naturally from the earliest issues of the Victorian period, through the Edwardian and Georgian keyplate designs to the pictorial definitives of the 1930s and beyond. Up to the 1960s such collections would have been well within bounds, but a glance at any stamp catalogue of the present day will reveal that both of these Caribbean islands have been unduly prolific, issuing hundreds of stamps and souvenir sheets every year.

It is doubtful whether anyone collects the stamps of Grenada or St Vincent from the Victorian issues right down to the present day. Traditional collectors would have lost interest long ago and confined their study to the earlier period. This does not mean to say that no one buys or collects the modern stamps of these territories. If that were the case they would have drastically reduced their output long ago.

The answer lies in the subject matter of the stamps. St Vincent was the first country to depict the ever-changing face of Michael Jackson and followed this up with numerous stamps devoted to Elvis Presley, Madonna and other icons of our era. Similarly Grenada (and its equally prolific dependency of the Grenadines, including individual islands such as Carriacou and Petite Martinique), have been very quick to sense the mood of the philatelic public and cater to every passing whim and ephemeral celebrity.

There must be very few subjects under the sun that have not appeared on postage stamps somewhere at some time or another. This is meat and drink to the person who collects stamps according to the theme or topic featured on them or relating to a particular event of global interest or perhaps a recurring sporting fixture like the Olympic Games and the World Cup.

Thematic collecting (as it is known in Britain - Americans prefer topical collecting) has always existed, although in the early period it was virtually confined to portraits of kings and queens or national emblems. By the 1920s, however, there were not only many more stamps available but the tendency to pictorialism had set in. In addition to the old favourites like portraits and heraldry, ships, railways and maps were popular themes, to which could be added stamps depicting aircraft as commercial aviation rapidly developed. In the 1930s some countries (notably Italy and Germany) realised the propaganda value of stamps which became not only more frequent but more dynamic in design. The advent of multicolour photogravure and offset lithography in the 1950s made stamps more eye-catching and appealing than ever and encouraged collectors to look at the images rather than the country of origin.

In 1949 the American Topical Association was founded and today the ATA boast over 10,000 members in more than 100 countries. It has made thematic collecting easier by publishing numerous check-lists depicting specific subjects as well as annual summaries of the world's stamps according to their subject. In more recent years Stanley Gibbons, Afinsa and other philatelic publishers have produced catalogues detailing the more popular themes, such as ships, aircraft, railways and birds, as well as some of the more esoteric subjects such as chess, seashells and fungi. Other subjects that are well catered for in catalogues and handbooks include the Red Cross, Scouts, Europa, Olympics and ball games (especially soccer).

Collectors are increasingly spoiled for choice. Time was when a philatelist with an interest in ornithology would have collected every stamp that depicted a bird, but with thousands to choose from most collectors will now concentrate on a particular species. Excellent collections have been formed of eagles, owls or even puffins.

Philatelists who turn to thematic collecting for a diversion often pick a subject which is associated with their career or some other hobby. Doctors tend to pursue medicine on stamps, from the great pioneers like Avicenna to Nobel laureates like Sir Alexander Fleming, or medicinal plants, combining an interest in botany or flowers. People who served in the armed forces can collect weapons and uniforms, badges and insignia, or stamps commemorating famous battles or campaigns (the 60th anniversary of D-Day this year has yielded a fine crop). Freemasons, Rotarians, Esperantists and youth leaders can all find lots of stamps associated with these interests.

You may not be able to afford to collect Old Masters or Post-Impressionists but you can have a most impressive picture gallery of your own since every aspect of fine art from Stone Age cave paintings to Surrealism and Dadaism has been reproduced on stamps. Numismatists can combine an interest in coins and medals with stamps depicting them, Sculpture, folk art, costume and handcrafts have all found generous coverage in philately. Even stamps themselves have been frequently reproduced, mainly in connection with the centenary or 150th anniversary of the first issues, but also to publicise philatelic exhibitions. Important anniversaries, such as the 75th, 100th and 125th anniversaries of the Universal Postal Union (founded in 1874), or the bicentenary of the birth and centenary of the death of Sir Rowland Hill (1795-1879) provided marvellous opportunities for a deluge of 'stamp on stamp' issues from all around the world.

Thematic collecting can be divided into three broad categories, each of which has a large following and even its own set of rules, at least so far as competitive exhibitions are concerned. Perhaps the most popular of these groups is known more specifically as subject collecting, since it consists of arranging stamps according to the subject depicted on them. If you decide to collect stamps showing horses, poultry, jet aircraft or fairy-tale characters most of the hard work has already been done as the ATA has separate chapters and has also published handbooks on these topics. If, however, you are of a more independent frame of mind and spurn the well-trodden paths, you will have to make a start with the latest edition of a whole world stamp catalogue and wade your way through, page after page, from Abu Dhabi to Zululand, making notes of all the stamps that seem to come within your scope. Even some of the philatelic software now available, which claims to produce a subject classification, will not provide a 100 per cent answer. Is the subject the main or subsidiary element of the design? If the latter, it will not be covered - but these are the stamps that make all the difference to the thoroughness of the collection.

You can break down your chosen subject into various categories. Thus horses would start with prehistoric horses, wild horses and horse relatives (zebras, asses and hybrid mules), mythological horses (centaurs, Pegasus, Sleipnir etc), horses in folklore and legend, and then move on to the horse in art from Roman friezes to equestrian portraits, horses in literature, work horses, war horses, post-horses, horses pulling carts, wagons and mail-coaches, chariot racing, steeplechasing, flat-racing and trotting, horses in sports such as polo buzkashi and guks, horses in tournaments, show-jumping, dressage, haute ecole and other equestrian events. You might even include saddlery and harness, horse ploughs and portraits of famous jockeys, race-courses and race trophies.

The second branch of thematics is known as purpose-of-issue collecting (sometime referred to in America as incidental philately). It ranges chronologically from the issues for Queen Victoria's Diamond Jubilee (1897) right down to the stamps now pouring forth for the Euro 2004 football championship and the Athens Olympics, and includes the issues made in tribute to Churchill, Kennedy and Diana, either as mourning stamps at or soon after death or the ongoing issues marking birth and death anniversaries.

The third category can be described as true thematics and this offers the greatest scope to the collector: the arrangement of stamps in such a way that they develop a theme or concept. In effect, the stamps are used to illustrate a story and tend to become merely incidental to the historical research undertaken by the collector. Among the more popular subjects are the history of the United States as portrayed on its stamps or Man's Conquest of Space, illustrated by stamps depicting rocketry from ancient China to the Columbia Shuttle disaster. The possibilities with this type of collection are endless and stamps can be used to illustrate everything from nuclear physics to Shakespeare quotations.

Nowadays it is not sufficient to use stamps (preferably mint) to make a point, but ancillary material such as first day and souvenir covers, maximum cards, meter marks and advertising labels (otherwise known as poster stamps).