THE CANONICAL MODEL FOR THE FORMATION OF TERRESTRIAL PLANETS AND GIANT PLANET CORES IMPLICITLY RELIES ON AN EARLY AND VERY EFFICIENT PHASE OF PLANETESIMAL GROWTH IN A GAS-RICH DISK. BUT AS THEORISTS HAVE KNOWN FOR DECADES NOW THERE ARE SOME FORMIDABLE OBSTACLES TO MEETING THAT REQUIREMENT. MANY OF THESE PROBLEMS AND POTENTIALLY THEIR SOLUTIONS ARE ASSOCIATED WITH THE GROWTH AND MIGRATION OF "PEBBLES" (MM/CM-SIZED SOLIDS) IN THE FIRST FEW MILLION YEARS OF A DISK'S LIFETIME. THAT IS FORTUITOUS SINCE THE THERMAL CONTINUUM EMISSION FROM THESE PARTICLES IN NEARBY DISKS CAN BE READILY DETECTED AND RESOLVED WITH RADIO INTERFEROMETERS. OBSERVATIONS OFFER SEVERAL LINES OF EVIDENCE THAT QUALITATIVELY SUPPORT THEORETICAL MODELS FOR THE GROWTH AND MIGRATION OF DISK SOLIDS BUT ALSO DEMONSTRATE A QUANTITATIVE TENSION WITH THE EFFICIENCY OF THAT EVOLUTION. RECENT OBSERVATIONS HAVE REVEALED PROMINENT LOCALIZED CONCENTRATIONS OF THAT RADIO CONTINUUM EMISSION ON SMALL (FEW AU) SCALES IN A FEW DISKS. THESE STRIKING SUBSTRUCTURES MANIFESTED AS ARCS SPIRALS AND CONCENTRIC RINGS AND GAPS IN THE SPATIAL DISTRIBUTIONS OF MM/CM-SIZED SOLIDS ARE LIKELY THE HALLMARKS OF A GENERAL SOLUTION TO THE TENSIONS WITH THE DISK MODELS. THEY SIGNAL THAT SOME MECHANISM SHEPHERDS SOLIDS INTO CONFINED AREAS BY SLOWING OR HALTING (AND PERHAPS ATTRACTING) THEIR MIGRATION. THEORETICAL WORK HAS SUGGESTED THAT THE CONDITIONS IN SUCH CONCENTRATIONS ARE FAVORABLE FOR TRIGGERING INSTABILITIES THAT CAN RAPIDLY CONVERT PEBBLES INTO MUCH LARGER PLANETESIMALS. VARIOUS MECHANISMS TO ACCOUNT FOR THE OBSERVED SUBSTRUCTURES HAVE BEEN PROPOSED INCLUDING LOCAL GAS PRESSURE MAXIMA INDUCED BY DYNAMICAL EFFECTS FLUID INSTABILITIES AND ABRUPT VARIATIONS IN THE PROPERTIES OF THE DISK MATERIAL. PERHAPS THE MOST TANTALIZING HYPOTHESIS IS THAT THESE DISKS HAVE BEEN DYNAMICALLY SCULPTED BY PREVIOUS GENERATIONS OF PLANET FORMATION. THESE SMALL-SCALE DISK SUBSTRUCTURES APPEAR TO BE FUNDAMENTAL PIECES TO THE PLANET FORMATION PUZZLE. WE ARE PROPOSING FOR SUPPORT TO (1) CONDUCT THE FIRST HIGH RESOLUTION SURVEY OF MM-WAVE CONTINUUM (AND MOLECULAR LINE) EMISSION FROM DISK SUBSTRUCTURES; (2) SYSTEMATICALLY QUANTIFY THEIR MORPHOLOGIES AMPLITUDES SCALES SEPARATIONS SYMMETRY AND DEPENDENCE ON HOST STAR PROPERTIES; AND (3) VET THE THEORETICAL MECHANISMS THAT HAVE BEEN PROPOSED TO PRODUCE THEM. THESE INVESTIGATIONS WILL DRAMATICALLY IMPROVE OUR UNDERSTANDING OF PROTOPLANETARY DISK STRUCTURES AND THE ORIGINS OF EXOPLANETARY SYSTEMS. THEY WILL ALSO PROVIDE GUIDANCE FOR DESIGNING DIRECT IMAGING SEARCH STRATEGIES AND INTERPRETING THE UPCOMING VAST MICROLENSING CATALOGS AVAILABLE FOR EXOPLANETS WITH LONG ORBITAL PERIODS FROM CURRENT AND FUTURE NASA FACILITIES.