  Since the seminal paper by Marzetta from 2010, Massive MIMO has changed from
being a theoretical concept with an infinite number of antennas to a practical
technology. The key concepts are adopted in 5G and base stations (BSs) with
$M=64$ full-digital transceivers have been commercially deployed in sub-6\,GHz
bands. The fast progress was enabled by many solid research contributions of
which the vast majority assume spatially uncorrelated channels and signal
processing schemes developed for single-cell operation. These assumptions make
the performance analysis and optimization of Massive MIMO tractable but have
three major caveats: 1) practical channels are spatially correlated; 2) large
performance gains can be obtained by multicell processing, without BS
cooperation; 3) the interference caused by pilot contamination creates a finite
capacity limit, as $M\to\infty$. There is a thin line of papers that avoided
these caveats, but the results are easily missed. Hence, this tutorial article
explains the importance of considering spatial channel correlation and using
signal processing schemes designed for multicell networks. We present recent
results on the fundamental limits of Massive MIMO, which are not determined by
pilot contamination but the ability to acquire channel statistics. These
results will guide the journey towards the next level of Massive MIMO, which we
call ``Massive MIMO 2.0''.
