The 1990s was when the contemporary mainstream sound--the modern refinement of jimmy Martin's band approach, as prefigured by the New South, Quicksilver, the Cardinals, et al.--really solidified and attained a dominant position in bluegrass, IMHO.

The 1970s were also a fruitful era, little known to those who've gotten into the music in recent years. We had various "progressive" flavors (e.g., the Country Gentlemen, the Seldom Scene, Country Gazette, the New Grass Revival), the early stirrings of today's mainstream sound (as mentioned above), and toward the end of the decade, the emergence of neotraditional bluegrass (e.g., the Dry Branch Fire Squad, Hot Rize, the Johnson Mountain Boys).

I tend to think about the development generationally.

Monroe, Flatt & Scruggs, The Stanleys, Reno & Smiley, and Jim & Jesse were the first generation. The Osborne Brothers, The Country Gentlemen, J.D. Crowe, The Seldom Scene, and The Bluegrass Alliance/Newgrass Revival seem to me to be the second generation.

The second generation began the process of adapting material from other musical genres. Also with the second generation, we saw the proliferation of lead guitar and the so-called "melodic" style of banjo, both of which were not prevalent during the first generation. The subsequent generations have continued the directions established by the second generation.

To me there’s no clear line between eras, it all just organically developed/expanded in scope. For each decade I’ll go with Monroe, early Flatt & Scruggs - 1940’s,, Flatt & Scruggs, Reno & Smiley , Stanley Brothers, 1950’s ,,, Country Gentlemen, Jim &Jesse, Osborne Brothers 1960’s ,,, Seldom Scene , JD Crowe & the New South , Country Gazette 1970’s,, early Alison Krauss & U.Stn. , Johnson Mountain Boys, Newgrass Revival,1980’s,, Laurie Lewis, then Blue Highway,, Lonesome River Band ,, Kenny and Amanda Smith ,, AKUS ,, Billy Strings ,, on & on