Insects were for a long time considered simple organisms with unvarying behavioural repertoires, incapable of complicated behavioural responses to changing environments and/or social conditions. However, nothing could be further from the truth; phenotypic plasticity is widespread in insect development, life history, physiology, and behaviour (Whitman and Ananthakrishnan 2009). Plastic responses to environmental and social conditions are actually central to the remarkable adaptability of insects, and have played a crucial role in their evolutionary histories (Moczek 2010; Simpson et al. 2011). Moreover, phenotypic plasticity in insects is not merely restricted to simple responses in metabolism or activity to abiotic factors such as temperature, but can be extremely elaborate, an illuminating example of which is the learning ability of honeybees (Giurfa 2007; Hammer and Menzel 1995; Menzel 1993; Menzel and Muller 1996). Insect mating systems are no exception to this pattern. This chapter explores the intrasexual variation in behaviours and morphologies found in insect mating systems. More specifically, we focus on the evolution of the alternative means by which individuals obtain fertilizations, generally referred to as ‘alternative mating tactics’, or ‘alternative mating phenotypes’ (AMPs). The first studies to describe what we can interpret today as cases of AMPs in insects date back to at least the 1930s (Salt 1937), but it was only in the 1970s that the number of studies reporting this phenomenon started to accumulate. In 1983, when the classic examples of AMPs in digger bees and scorpionflies were reviewed by Thornhill and Alcock (1983), approximately 50 cases of AMPs in insects were already known, a number that has now surpassed the 200 mark. Here, we review the theoretical and empirical advances that have been made in this area since Thornhill and Alcock’s volume. We start by describing two illustrative systems in detail, gryllid field crickets and onthophagine dung beetles. These two groups were chosen because their reproductive biology is well known, and because the contrasting degrees of behavioural plasticity and morphological specialisation between alternative phenotypes in these two groups illustrate the diversity of AMPs that has evolved in insects. We then discuss the genetic models that have been proposed to account for the evolution and maintenance of such dimorphisms, before reviewing the occurrence of AMPs in insects more generally. Finally, we discuss the relatively limited evidence for AMPs in female insects, a somewhat new and very promising area for future research.