  We address the question of aggregating the preferences of voters in the
context of participatory budgeting. We scrutinize the voting method currently
used in practice, underline its drawbacks, and introduce a novel scheme
tailored to this setting, which we call "Knapsack Voting". We study its
strategic properties - we show that it is strategy-proof under a natural model
of utility (a dis-utility given by the $\ell_1$ distance between the outcome
and the true preference of the voter), and "partially" strategy-proof under
general additive utilities. We extend Knapsack Voting to more general settings
with revenues, deficits or surpluses, and prove a similar strategy-proofness
result. To further demonstrate the applicability of our scheme, we discuss its
implementation on the digital voting platform that we have deployed in
partnership with the local government bodies in many cities across the nation.
From voting data thus collected, we present empirical evidence that Knapsack
Voting works well in practice.
