The research work described in this thesis, the environmentally friendly novel "Microwave Assisted Sub-Critical water (MASCW)" technology for particle engineering of active pharmaceutical ingredients and excipients was developed. The present novel technology MASCW process is described as green technology as water is used as the solvent medium and microwave energy as external source of heat energy for generation of a particle with different morphological and chemical properties. In MASCW process supersaturated solution of APIs is prepared by dissolving solute in water at high temperature and pressure conditions. Upon rapid and controlled cooling, based on the aqueous solubility of solute, solute/solvent concentration and dielectric constant of water rapid precipitation of API with narrow particle size distribution occurs. Using paracetamol (pca) as API moiety understanding of the mechanism of MASCW crystallisation process was investigated. The effect of different process and experimental parameters on crystallisation pathway and end product attributes were analysed. Correlation between the degree of supersaturation concentration of pca solution against temperature and pressure parameters was explained by generating binary phase diagram. Determination of polymorphic transformation pathway of pca from form I (stable) to form II metastable polymorphs in solution was analysed using Raman spectroscopy. The difference between conventional heating and subcritical treatment was explored by determining the change in the solvent dielectric constant and solubility of hydrophobic API molecule. Based on the process understanding results, this technology was further implemented to explore its application in generating phase pure stable and metastable cocrystal phase. Based on the solubility of API and cocrystal former congruent (CBZ/SAC, SMT/SAC, SMZ/SAC) and incongruent (CAF/4HBA) cocrystal pairs were selected. For the first time generation of anhydrous phase of CAF: 4HBA cocrystal in 1:1 stoichiometric ration was reported and generation of metastable cocrystal phase of CA CBZ: SAC form II was reported. The application of this technology was explored in generating phase pure metastable polymorph of paracetamol which retain higher compressibility and dissolution rate. The potential of MASCW micronisation process, theophylline is used as the model component to produce micro sized particles for pulmonary drug delivery system via dry powder inhaler (Foradil inhaler). The results demonstrate that the THF particles generated using MASCW process displayed greater aerodynamic performance compared to conventional spray-dried THF sample. In the final chapter, synthesis of inorganic biomaterial (nano crystalline hydroxyapatite) was reported for the first time and the prospects of combining API like ibuprofen (IBU) with a biologically active component like nano-crystalline hydroxyapatite (HA) through hydrogen bonding was mechanistically explained using X-ray diffractometer and spectroscopic techniques.