Variability in biology contributes to adaptability and fitness of a species. Even though variability occurs ubiquitously in biology, the origins and consequences are often poorly understood. Biological variability could be caused by functional stochasticity that is selected by evolution, or a more organized molecular or cellular process at its basis. In this thesis, a specific form of variability, which is represented by the occurrence of two distinct forms of a species, tissue, or population, will be discussed. This form of variability, called dimorphism, derives from the Greek word dimorphos (from di- meaning two and morphe meaning shape or form). To try and gain a better understanding of the occurrence and function of dimorphism in biology, the topic will be considered in three sub-themes: sexual dimorphism (A), bilateral asymmetries (B), and variability in behavior (C). These sub-themes will be addressed using nematodes as a model organism; they are among the simplest multicellular organisms, making them ideal to study this subject on a cellular, organism, and population level. A) First, we studied the transcriptomic variability underlying the sexually dimorphic Caenorhabditis elegans. In Chapter 2, we show the varying gene expression patterns along the anteroposterior axis and address the functional role of differential gene expression in the male reproductive tract. B) Second, we discussed the left-right asymmetrical migration of Q neuroblasts in C. elegans. As described in Chapter 3, several transmembrane proteins are involved in initial Q neuroblast migration. Even though these transmembrane proteins have a role in Q cell migration, they are not the origin of symmetry breaking. We, therefore, addressed the possible origin of Q neuroblast symmetry breaking in Chapter 4. Initial Q cell migration is required for proper Wnt signaling response and the directional onset migration of the Q daughter cells. The role of the Hox transcription factor MAB-5 in the posterior migration of QL daughter cells is described in Chapter 5. Moreover, the migration termination of the anterior migrating QR daughter cell QR.pa is addressed in Chapter 6. C) Third, we focused on behavioral variability in populations of nematodes. In Chapter 7, we studied the role of variable gene expression in salt-preference in C. elegans. To conclude, the research shown in this thesis was put in a broader perspective in the general discussion where the findings were summarized and discussed. The overall observed function, occurrence, and origins of variability in biology are addressed in the context of adaptation and evolution.