I develop a model linking specific microinteraction dynamics between members of different institutional fields and the genesis of new practices. The model centers on the concept of interstitial spaces—that is, small-scale settings where individuals from different fields interact occasionally and informally around common activities to which they devote limited time (e.g., hobbyist clubs, hangouts, workshops, meet-ups). I argue that the features of interstitial spaces (e.g., their institutional diversity and their occasional and informal nature) facilitate the individuals interacting in these settings to temporarily break free from existing institutions and experiment collectively with new activities and ideas. However, these very same features hinder the constitution of such new activities and ideas into new practices. I identify two microlevel conditions that enable the new activities and ideas developed in interstitial spaces to be constituted into new practices: the emergence of successful interaction rituals, and the presence of catalysts sustaining others' interactions and assisting the construction of shared meanings.