There are various environments where conventional wireless communications using electromagnetic (EM) waves is not feasible or detrimental. For example, at the macro-scale, EM waves suffer from high propagation path loss in pipe networks and tunnels. Moreover, inside the human body (micro/nano-scale) the EM radiation could result in unwanted effects on health. Inspired by nature (e.g., communications between cells), the information exchange using chemical signals has been proposed as a promising solution for the aforementioned environments, which is referred to as molecular communications (MC). Due to its ultra-high efficiency and bio-compatibility it is especially suited for communications at nano-scale, such as inside the human body. In MC, the information is encoded using the concentration, release time, and type of molecules and the propagation from transmitter to receiver can be either passive (only diffusion of molecules) or active (diffusion-advection, molecular motors).