Ontologies that explicitly identify objects, properties, and relationships in specific domains are essential for collaborations that involve sharing of data, knowledge, or resources among autonomous individuals. Against this background, this paper motivates the need for collaborative environments for ontology construction, sharing, and usage; identifies the desiderata of such environments; and proposes package based description logics (P-DL) that extend classic description logic (DL) based ontology languages to support modularity and (selective) knowledge hiding. In P-DL, each ontology consists of packages (or modules) with well-defined interfaces. Each package encapsulates a closely related set of terms and relations between terms. Together, these terms and relations represent the ontological commitments about a small, coherent part of the universe of discourse. Packages can be hierarchically nested, thereby imposing an organizational structure on the ontology. Package-based ontologies also allow creators of packages to exert control over the visibility of each term or relation within the package thereby allowing the selective sharing (or conversely, hiding) of ontological commitments captured by a package.