Roads impose several types of negative impact on landscapes and biodiversity, but may also favor some organisms by providing habitats and dispersal corridors. To prioritize, plan, design, and perform activities for promoting biodiversity in road verges, it is essential to understand which key environmental factors contribute to forming different types of roadside habitats. In this chapter, we explore relationships between biodiversity and environmental factors in road verges based on a literature review with a primary focus of vascular plants and arthropods. Roadside literature indicates a number of interacting ecological factors, which together form the roadside habitat and determine community composition. These key factors can be assigned to three groups: (1) ecological conditions such as soil, topography, and microclimate, (2) ecological processes such as vegetation succession and disturbance or management of ground and vegetation, and (3) the surrounding landscape. Based on the identified key factors, we suggest an ecological classification of roadside habitats into four major groups, namely successional roadsides, dry roadsides, tallgrass roadsides, and meadow roadsides. Trees and shrubs can occur in all groups, for example as tree avenues or hedgerows, resulting in a cross-cutting subgroup: successional/dry/tallgrass/meadow roadsides with trees and shrubs.