.Net Framework (do we need all versions?)

Anyone can explain me if i should get 3.5 only or get all versions and install in order?

Please tell me wich versions should i get ???

Thanks in advance

That pretty much depends on the programs your running.
.Net is only required if you use a program that needs it.

I got some older version but for sure i will let them on. My Windows installed 3.5 SP1.