  This is brief review of acceleration of electrons in plasma wakefields driven
by either intense laser pulses or particle beams following lectures at the 2019
CERN Accelerator School on plasma accelerators, held at Sesimbra, Portugal. The
commonalities between drivers and their strength parameters and operating
parameter regimes for current experiments in laser wakefield acceleration
(LWFA) and beam driven plasma wakefield acceleration (PWFA) are summarized.
Energy limitations are introduced, including the dephasing and depletion
lengths for lasers, and the transformer ratio for beam driven plasmas. The
concept of the wake Hamiltonian is introduced and the resulting particle orbits
are identified in phase space, which illustrates how the peak energy and energy
spread of accelerated electrons are determined.
