The Tretyakov Gallery is a must see stop in Moscow.
The State Tretyakov Gallery possesses a unique collection of Russian art which includes masterpieces which span a period of a thousand years and much of it is unknown to Western eyes--or at least to my Western eyes.
The Gallery was founded by a Russian merchant and patron of the arts Pavel Tretyakov. He donated his collection to the city of Moscow in 1892. The State Tretyakov Gallery has since become a world-famous museum. Nowadays it contains more than 170,000 works by Russian artists from early religious paintings to modern art. The very best parts are the medieval Russian icons and the ninteenth century Russian painters. Icons are an acquired taste, but my spouse, who is not at all in love with religious artifacts, finds them fascinating. The Tretyakov collection is impressive but it can be hard it's hard for harder core art lovers to get over the derivative, Italian-influenced portraits and landscapes of the 18th Century, but the rejection of the Imperial Academy's restrictive diktats and the attempt to create a national art for the people that gained momentum as the 19th century progressed produced some fascinating results. Fans of Russian literature can entertain themselves by seeing how the concerns and ideals of the great 19th century writers were reflected by the artists of the same period.
The art is one feature of the gallery--the other is the house in which it is housed. Simply beautiful and a wonderful place to spend an afternoon. I have never been much of one to hire a guide for a museum, but if you knwo nothing about Russian art, it would be well worth your while to get someone to help show up some of the wonderful highlights of the place.