Etoposide has been shown to be effective in the treatment of testicular and small-cell lung cancers, lymphoma, leukemia and Kaposi's sarcoma. Several clinical investigations have suggested that the prolonged maintenance of greater than 1 $\mu$g/ml concentration in plasma would provide better therapeutic response in patients. Thus use of a sustained/controlled release formulation of etoposide was indicated. This investigation focused on the potential for the development of a sustained/controlled release dosage form of etoposide for a 7-15 day delivery using selected polylactic and polylactic-co-glycolic acid polymers. During the course of studies involving the enhancement of aqueous solubility of etoposide in our laboratory evidence of a potential thermally induced polymorphic transition was detected. Therefore, further characterization of this phenomenon was also included in this investigation. Thermal behavior of etoposide was characterized by differential scanning calorimetry, thermal gravimetric analysis, X-ray diffractometry, mass spectroscopy, IR spectra and HPLC analyses. A method for the preparation of micromatrices of etoposide was developed utilizing a suspension and solvent evaporation technique. DSC, IR and NMR investigations did not indicate any potential etoposide-polymer interaction. Etoposide I, a monohydrate, underwent a dehydration reaction between 85-115$\sp\circ$C to yield Etoposide Ia, which upon further heating melted at 198$\sp\circ$C and crystallized to a new polymorph, Etoposide IIa at 206$\sp\circ$C. Etoposide IIa was found to melt at 269$\sp\circ$C and converted to its hydrated form, Etoposide II when exposed to atmosphere at room temperature. The polymorphic transition was found to be irreversible and monotropic. Etoposide I, the currently marketed drug was used in all delivery systems examined. Formulation studies with polylactic acid polymers indicated that the molecular weight of the polymer was a key parameter in influencing the percent of drug entrapped in the micromatrices, particles size distribution and the drug release profiles. Glycolide-containing polymers demonstrated control of etoposide release only at low drug loadings: larger micromatrices showing better control. Polylactic acid 50,000 at 1:5 and 1:15 drug to polymer ratios exhibited maximum rate of drug release of 1.57 mg/hr. At this release rate, a delivery system containing 350 mg of etoposide could be expected to maintain a plasma concentration of 1.08 $\mu$g/ml over a period of 7 days. Additionally, drug release profile of polylactide-co-glycolide (85:15, 75-180 $\mu$m) microsphere formulation with 1:10 drug to polymer ratio, was found to be more appropriate for a 15-day release system based upon 700 mg of etoposide.