user8291: It seems to me that after the popular move order 
[fen ""]

1. d4 Nf6 2. c4 e6 3. Nf3 d5 4. Nc3 Be7 

the most popular option - at least at top GM level - is currently 5. Bf4. Yet, back in the 1980s and 1990s, 5. Bf4  was not considered a particularly challenging way to fight the Queen's Gambit Declined. Every strong GM was playing 5. Bg5 (or the Exchange Variation). Who revitalized  5. Bf4, and why is it considered a strong / challenging move nowadays?

user4588: One of the possible reasons why 5.Bf4 has gained popularity at the very top-level, is because white seems to have difficulties obtaining an advantage after 5.Bg5.
The critical variation is 5....h6 6.Bh4 O-O 7.e3 Nbd7 8.Rc1 c5 9.dxc5 Nxc5 10.cxd5 g5!, which occured for the first time in Wojtaszek-Kramnik (2015).
Moreover, black can also play 9....dxc4, which happened in another famous Kramnik game: Matlakov-Kramnik (2015).
After 8....c5, white's main alternative is 9.cxd5. In this line too, there is an impressive Kramnik win: Salem-Kramnik (2014).
To answer the question "Who revitalized 5.Bf4?": Kramnik, by posing some questions as black in the 5.Bg5 line.


      [StartPly "8"]

      [FEN ""]
      1.d4 Nf6 2.c4 e6 3.Nf3 d5 4.Nc3 Be7 5.Bg5 (5.Bf4) h6 6.Bh4 O-O 7.e3 Nbd7 8.Rc1 c5 9.dxc5 (9.cxd5) Nxc5 (9...dxc4) 10.cxd5 g5




8291: <p>It seems to me that after the popular move order </p>

<pre><code>[fen ""]

1. d4 Nf6 2. c4 e6 3. Nf3 d5 4. Nc3 Be7 
</code></pre>

<p>the most popular option - at least at top GM level - is currently 5. Bf4. Yet, back in the 1980s and 1990s, 5. Bf4  was not considered a particularly challenging way to fight the Queen's Gambit Declined. Every strong GM was playing 5. Bg5 (or the Exchange Variation). Who revitalized  5. Bf4, and why is it considered a strong / challenging move nowadays?</p>
 4588: <p>One of the possible reasons why 5.Bf4 has gained popularity at the very top-level, is because white seems to have difficulties obtaining an advantage after 5.Bg5.</p>

<p>The critical variation is 5....h6 6.Bh4 O-O 7.e3 Nbd7 8.Rc1 c5 9.dxc5 Nxc5 10.cxd5 g5!, which occured for the first time in <a href="http://www.chessgames.com/perl/chessgame?gid=1791496" rel="nofollow noreferrer">Wojtaszek-Kramnik (2015)</a>.
Moreover, black can also play 9....dxc4, which happened in another famous Kramnik game: <a href="http://www.chessgames.com/perl/chessgame?gid=1811839" rel="nofollow noreferrer">Matlakov-Kramnik (2015)</a>.
After 8....c5, white's main alternative is 9.cxd5. In this line too, there is an impressive Kramnik win: <a href="http://www.chessgames.com/perl/chessgame?gid=1779836" rel="nofollow noreferrer">Salem-Kramnik (2014)</a>.</p>

<p>To answer the question "Who revitalized 5.Bf4?": Kramnik, by posing some questions as black in the 5.Bg5 line.</p>

<pre>
<code>
      [StartPly "8"]

      [FEN ""]
      1.d4 Nf6 2.c4 e6 3.Nf3 d5 4.Nc3 Be7 5.Bg5 (5.Bf4) h6 6.Bh4 O-O 7.e3 Nbd7 8.Rc1 c5 9.dxc5 (9.cxd5) Nxc5 (9...dxc4) 10.cxd5 g5

</code>
</pre>
