  An active colloid is a suspension of particles that transduce free energy
from their environment and use the energy to engage in intrinsically
non-equilibrium activities such as growth, replication and self-propelled
motility. An obvious example of active colloids is a suspension of bacteria
such as Escherichia coli, their physical dimensions being almost invariably in
the colloidal range. Synthetic self-propelled particles have also become
available recently, such as two-faced, or Janus, particles propelled by
differential chemical reactions on their surfaces driving a self-phoretic
motion. In these lectures, I give a pedagogical introduction to the physics of
single-particle and collective properties of active colloids, focussing on self
propulsion. I will compare and contrast phenomena in suspensions of `swimmers'
with the behaviour of suspensions of passive particles, where only Brownian
motion (discovered by Robert Brown in granules from the pollen of the wild
flower {\it Clarkia pulchella}) is relevant. I will pay particular attention to
issues that pertain to performing experiments using these active particle
suspensions, such as how to characterise the suspension's swimming speed
distribution, and include an appendix to guide physicists wanting to start
culturing motile bacteria.
