Their line-up of Progressive period:  

Mark 1 (and the only) - Keith Emerson (Keyboards), Greg Lake (Vocal, Guitar and Bass) and Carl Palmer (Drums).

Not like other crowed Prog Rock bands, ELP were just a 3 people group, but under the huge keyboards arrangement, ELP was enough to generate complicate and charming sound. 

ELP was said to be a super group of Prog Rock, as Keith came from The Nice, Greg came from King Crimson and Atomic Rooster. And they formed in the growing time (1970) of Prog Rock, with the supported of radio and press, they rose very fast and got lots of audience.

You can also say the music ELP played is Classical Rock, as Keith was trained classically and ELP played lots of classical pieces with their new arrangement in rock style. As a trio, ELP could spread the hot and power like traditional guitar lead band, mostly because of Keith's showmanship, flinging knives at his keyboard, moving his synths to sound feedback, etc. Also Greg with Carl's tight and powerful drumming provided a solid sound rhythms session. 

Their most famous rearrange work surely must be the Pictures at an Exhibition that originally composed by the Russia composer, Modest Mussorgsky (1839 ~ 1881). It was first released as a live record in 1972, plus a session of Pyotr Tchaikovsky (1840 ~ 1893)'s Nutcracker as Nutrocker. 

And their most appreciated work was Brain Salad Surgery (1973). It presented the best and the various side of ELP. Like normal piece, Jerusalem and Still You Turn Me On to long Karn Evil 9. But compared with other Progressive groups, the lyrics was ELP weakest link, so sometimes will ask for help from King Crimson Pete Sinfield.

The last Progressive records they made was Works (1977), that's a 2LPs, 4 sides album. While each of them took one side individually and worked together in the last side. It's true that ELP found their direction right but just kept digging in without new ideas, so very soon, their music became dried.  

As the time Punk Exploded, ELP was not generally accepted anymore. John Peel even said their huge synths setup on stage is a wasted of electricity. But no doubt their experienced of playing orchestral pieces in a rock trio remained in the music history.  

The way they worked in Works also showed they were no longer a band and going to split, and they did, after the releasing Love Beach (1978) that was an album they owed to the records company.