Is it busy in the summer?

Main season is appr. mid of June until mid of August. After this period still a lot of travelers are on their way.

I've traveled to most countries in Europe since 2015. Norway in 2022 (June 15 - July 8) certainly had tourists, but it was nothing like what I've run into in popular cities farther south. Norway is quite expensive, which is probably one reason it's not overrun. Coastal cities may have cruise ships in port, however.

From the planning perspective, the thing to keep in mind is that some small towns (including Flam on the highly recommended Norway-in-a-Nutshell route from Oslo to Bergen) have insufficient accommodations to meet the peak-season demand, so early booking is really important. I don't think you'll run into a problem in Oslo or even Bergen unless your dates align with a popular local special event.

Individual trains traveling west from Oslo can sell out, and even if they do not, the fare will creep up as others buy the cheapest tickets (a pattern common in Europe). I think the Flamsbana train from Myrdal down to Flam can also sell out.

I think that you might actually be too late for some activities in late August. I know that our rubber boat tour in Flam in late August was one of the last of the season. There are plenty of other things to do and it wasn’t cold or snowy; but some of the mountainous places were autumnal rather than summerish.

We loved Norway and would return any time.

Safe travels.

We were there for 18 days in September of 2023. There were places that were really busy (Flam, Stegastein and Geiranger) and other places that were really quiet (lodging near the Atlantic Road). We didn't find anything shut down for the season while we were there (except the ski museum in Morgedal).

We are looking at going back in 2026, and for our schedule, the last half of August into September works the best for us.

Thanks for all the replies!
I'm still thinking of some time in August, as air fares from Seattle are way cheaper than June, and I hear July is rainy.

Check your weather website of choice, because the monthly averages shown in the Wikipedia articles on Oslo and (especially) Bergen indicate August is typically rainier than June and July. Bergen is one of Europe's wettest cities, and it's even wetter in September than in August.

If you're thinking doing the Norway in a Nutshell route, either the package or DIY, take a look at the cruise port schedule for Flåm and try to choose a date when there are not multiple big cruise ships in town. That will help alleviate some of the crowds .