  Biofilms are complex, self-organized consortia of microorganisms that produce
a functional, protective matrix of biomolecules. Physically, the structure of a
biofilm can be described as an entangled polymer network which grows and
changes under the effect of gradients of nutrients, cell differentiation,
quorum sensing, bacterial motion, and interaction with the environment. Its
development is complex, and constantly adapting to environmental stimuli. Here,
we review the fundamental physical processes the govern the inception, growth
and development of a biofilm. Two important mechanisms guide the initial phase
in a biofilm life cycle: (\emph{i}) the cell motility near or at a solid
interface, and (\emph{ii}) the cellular adhesion. Both processes are crucial
for initiating the colony and for ensuring its stability. A mature biofilm
behaves as a viscoelastic fluid with a complex, history-dependent dynamics. We
discuss progress and challenges in the determination of its physical
properties. Experimental and theoretical methods are now available that aim at
integrating the biofilm's hierarchy of interactions, and the heterogeneity of
composition and spatial structures. We also discuss important directions in
which future work should be directed.
