Crystal seeding is the process of adding homogeneous or heterogeneous crystals to a crystallising solution to nucleate and/or grow more crystals. Seeding has emerged as one of the most critical steps in optimising the crystallisation process (O’Sullivan B, Smith B, Baramidze G, Recent advances for seeding a crystallization process. Mettler Toledo Auto-Chem, Columbia, 2012). An aptly designed seeding technique would ensure product reproducibility between batches or over time. This is achieved primarily by controlling the crystal size distribution and polymorphism of the crystals that are formed. In this chapter, aspects of crystal nucleation, the importance of seeding and crystallisation methods employed will be discussed.