Introduction Uncertainty is an unavoidable product of any encounter between two or more animals. Although the level of uncertainty may vary as a function of many factors – including species, sex, age and condition – even at its minimum it still presents a challenge that must be overcome over the course of every animal’s life (Dall, 2010). This process is facilitated by the use of socially acquired information, which one animal gleans by observing the behaviour of another (Danchin et al., 2004; Dall et al., 2005), thereby reducing its prior uncertainty before taking definitive action (Dall, 2010). Socially acquired information may be produced inadvertently (i.e. ‘public information’ and ‘social cues’), or actively broadcast by one individual in order to influence the behaviour of others (i.e. a ‘signal’) (Danchin et al., 2004; Dall et al., 2005). This chapter focuses on the latter, which forms the basis of animal communication. Animal communication has been observed in both terrestrial and aquatic habitats and, so far as we are aware, in all taxa in which it has been investigated (Lishak, 1984; Lugli, Yan & Fine, 2003; Marler & Slabbekoorn, 2004; Belanger & Corkum, 2009; Caro, 2009; Houck, 2009; Mäthger et al., 2009; Bruschini, Cervo & Turillazzi, 2010; Costa-Leonardo & Haifig, 2010; Haddock, Moline & Case, 2010; Wyatt, 2010; Thiel & Breithaupt, 2011). Communication occurs between both con- and heterospecifics (Rabin et al., 2003; Magrath, Pitcher & Gardner, 2007; Lea et al., 2008; Pope & Haney, 2008; Touhara, 2008; Shabani, Kamio & Derby, 2009; Bruschini et al., 2010) and, if one relaxes the definition of ‘animal communication’ to include any information transfer where either the signaller or receiver is an animal, it also occurs across phyla (Schaefer, Schaefer & Levey, 2004; Gera & Srivastava, 2006; Raguso, 2008).