So I've been getting into jazz improv lately. I've been mainly using arpeggios and following the chords, and I've done some work with modes. I was interested in the melodic minor though. I've heard tons of conflicting stuff about it though. So how exactly do you use it in improv, and when do you know to use it?
You're supposed to only use it when ascending the scale, and use natural minor when descending.
Wow, that's confusing. How are you supposed to think about that when you're improvising?
The only differences are the 6 and 7. Basically, if you're going down from the root, go to the b7. If you're going up to the root, use the 7. If you're going down to the 5, use the b6. If you're going up from the 5, use the 6.
Quote by blue_strat
You're supposed to only use it when ascending the scale, and use natural minor when descending.

Not in jazz, though. That's the more classical use. In jazz, you go 1 2 b3 4 5 6 7 both up and down.

You can use it over mmaj7 chords (1 b3 5 7 and extensions) but those are usually just passing chords, so this would really limit how much you can use them. However, you can also use them sometimes over m7 chords for an outside sound, and switch between dorian and melodic minor to get chromatic sequenced ideas and outline the V of the chord you're on and other wonderful things. (if you want examples of such things, I could probably think something up) You shouldn't overuse this method, though; pick your spots.

The modes also present some interesting opportunities. The 4th mode is lydian dominant, I believe, which is like the lydian mode with a flat seventh, so it could work over 7#11 chords. The seventh mode is the altered scale: 1 b2 b3 b4 b5 b6 b7. Note that the b4 could function as a major 3rd, and you might consider the b3 to be a #2 in this case. So, this altered scale could work over alt chords (although john knows more about this sort of thing than me, so this might not be the best scale choice. But it works).
(Slightly outdated) Electronic and classical compositions by m'self: Check 'em out
That's really not in practice these days, especially in jazz. The way to really implement it is to find the chord scale for melodic minor. Here it is for A melodic minor:
Am(maj7) - Bm7 - Cmaj7+ - D7 - E7 - F#m7b5 - G#7alt

So anytime that you see any of these chords, you can play A melodic minor. As far as the D7 and E7 are concerned: there are many choices of scales to play over dominant chords, so someone might give the sign to play using melodic minor if the above chords were specified as D7#11 and E7b13, respectively.

Another thing to note is that for something like the F#m7b5, A melodic minor is a normal choice for a jazz improviser, though you could also play G major, since the chord is found in the G major chord scale as well. It's also found in E harmonic minor and E harmonic major.

I think a major part of being a jazz improviser is being able to switch between different tonalities (major, melodic minor, harmonic minor, harmonic major, diminished, whole-tone, etc.). This is one way that jazzers combat running out of ideas; when you switch to a new scale, it carries with it a different feel and new ideas.

When you're practicing, write down the lines that you like. After you start accumulating a decent number of lines, start playing around with their rhythm and feel, and transpose them and play them in different keys, scales, and tonalities. After a while, you should have assimilated the lines and be able to use them in your improvisations.

Edit: That first line was to the guy above psychodelia.
Thanks a lot you two. I'm going to have to read through both your posts several times to really digest it but I already understand some of it.
Here's a fairly simple approach:

The predominant (haha dominant, you'll see...) way the melodic minor is used
in Jazz is over a functioning dominant chord. Dominants are generally the places
you can take the most "liberties". A functioning dominant is one that resolves
to other chords (like the V7 in a ii7-V7-Imaj7 progression).

Long story short: these are the common substitions:

Up a 1/2 step (ie Ab Melodic Minor over G7)
Down a 1 step (F melodic Minor over G7)
Up a 4th (C Melodic minor over G7)

Naturally, those are modes the Melodic Minor, but just described in reference to
the dominant.

The main one you should probably focus on is up 1/2 step -- that's the "altered scale"
which is the most used it gives you all altered tonalities -- b9/#9/b5/#5.