In-situ observations of mixed-phase clouds (MPCs) regularly reveal that ice crystal number concentrations (ICNCs) are orders of magnitude higher than ice-nucleating (IN) particle concentrations. This discrepancy has often been attributed to the influence of missed IN sources and secondary ice production (SIP) processes, but only recently considerable work has been invested towards better understanding the role of SIP. Here we present an overview of the main SIP processes and impacts that have been identified for globally-important cloud types (including polar, orographic and other cloud systems), and preset efforts to parameterize and study their impacts through a combination of observations and simulations for for a variety of case studies over the years. An emphasis will be placed on what has been learned and what critical knowledge gaps remain for weather-prediction and climate models.