The integration of nanoparticles with proteins is of high scientific interest due to the amazing potential displayed by their complexes, combining the nanoscale properties of nanoparticles with the specific architectures and functions of the protein molecules. The nanoparticle-protein complexes, in particular, are useful in the emerging field of nanobiotechnology (nanomedicine, drug delivery, and biosensors) as the nanoparticles having sizes comparable to that of living cells can access and operate within the cell. The understanding of nanopartide mteraction with different protein molecules is a prerequisite for such applications. The mteraction of the two components has been shown to result in conformational changes in proteins and to affect the surface properties and colloidal stability of the nanopartides. In this feature article, our recent studies exploring the driving interactions in nanopartide-protein systems and resultant structures are presented The anionic colloidal silica nanoparticles and two globular charged proteins [lysozyme and bovine serum albumin (BSA)] have been investigated as model systems. The adsorption behavior of the two proteins on nanoparticles is found to be completely different, but they both give rise to similar phase transformation from one phase to two phase in respective nanopartide-protein systems. The presence of protein induces the short-range and long-range attraction between the nanoparticles with lysozyme and BSA, respectively. The observed phase behavior and its dependence on vanous physiochemical parameters (e.g, nanopartide size, ionic strength, and solution pH) have been explained in terms of underlying interactions.