Molecular Communication via Diffusion (MCvD) is an effective and energy efficient method for transmitting information in nanonetworks. In this chapter, we focus on the modulation techniques in a diffusion-based communication system. We mainly assume the first hitting process for the reception of the signal and it affects the design of the modulation techniques. As observed in the nature, whenever an information carrying molecule hits to the receiver it is removed from the environment. These information molecules are called messenger molecules and can be of many types of chemical compounds such as DNA fragments, proteins, peptides or specifically formed molecules. Information is modulated on one or more physical properties of these molecules or the release timing. In this chapter, we mention four novelmodulation techniques, i.e., concentration, frequency,molecular-type, and timing-based modulations forMCvD in a single transmitter and single receiver environment. We also exemplify a systematic realization for molecular-ratio-based modulation using isomers as messenger molecules for MCvD. Next, we compare the pros and cons of the modulation techniques for an absorbing receiver that are studied in the literature. Knowing the workings and the properties of these modulation techniques enables us to use them in combination whenever it is possible.