1 Introduction.- 1.1 Definitions and Historical.- 1.2 The Root Environment.- 1.3 Significance and Purpose of Rhizosphere Investigations.- 2 The Structure and Function of Roots.- 2.1 Introduction.- 2.2 Root Morphology.- 2.3 Root Structure.- 2.4 Root Physiology.- 2.5 The Root-Soil Boundary.- 2.6 Root Exudation.- 3 Root Exudates.- 3.1 Introduction.- 3.2 Methods of Collection and Analysis.- 3.3 Qualitative and Quantitative Nature of Exudates.- 3.4 Sites of Exudate Release.- 3.5 Factors Affecting Exudation.- 3.6 Sloughed Organic Matter.- 4 Rhizosphere Populations.- 4.1 Introduction.- 4.2 Isolation and Enumeration of Microorganisms.- 4.3 Populations of the Microflora.- 4.4 Qualitative Changes in the Microflora.- 4.5 Factors Affecting Populations of the Microflora.- 4.6 Populations of the Rhizosphere Fauna.- 5 Microbial Interactions.- 5.1 Introduction.- 5.2 Growth-Promoting Interactions.- 5.3 Growth-Inhibiting Relationships (Antagonism).- 6 Rhizosphere in Relation to Plant Nutrition and Growth.- 6.1 Introduction.- 6.2 Nutrient Availability and Uptake.- 6.3 Nonsymbiotic Nitrogen Fixation.- 6.4 Symbiotic Relationships.- 6.5 Microbial Metabolites.- 6.6 Plant Effects on Other Plants.- 7 Rhizosphere Relation to Plant Disease.- 7.1 Introduction.- 7.2 Pathogen Populations.- 7.3 Growth and Survival in the Rhizosphere.- 7.4 Pathogenesis.- 7.5 Pathogenesis Without Parasitism.- 7.6 Disease Control.- 8 Current Trends and Projected Emphasis.- 8.1 General Overview.- 8.2 Status of the Technology.- 8.3 Relating to Plant Growth.- 8.4 Opportunities in Plant Pathology.- 8.5 Focus: The Rhizosphere and Crop Production.- References.