Secondary nucleation is the dominant nucleation mechanism in industrial crystallizers. This work reviews secondary nucleation as it occurs in solution-based crystallization of small organic/inorganic compounds. The review elucidates various mechanisms along with the experimental and analytical techniques used to study them. Based on the review, a refined classification of secondary nucleation mechanism is proposed. Mechanistic models for various secondary nucleation mechanisms are then discussed at length. The review ends with an overview of the metastable zone width for secondary nucleation. The review will be useful to researchers and engineers working on bulk crystallization of small molecules to gain in-depth understanding of secondary nucleation.