The present report describes the main results and achievements regarding the electrochemical and photoelectrochemical investigation of as-formed and annealed TiO 2 nanotubes (TNTs). Titanium oxide nanotubes were prepared by anodic oxidation of titanium within fluoride containing ethylene glycol and potassium sulfate solutions. Using an electrochemical approach, it has been shown that as-formed nanotubes are not stable on the substrate due to the dissolution of fluoride rich layer between and beneath them resulting in formation of a new barrier layer between TNTs and metal substrate by re-anodization in aqueous solutions. This new anodic barrier layer, which forms inevitably by any anodic polarization of the unstable interface of titanium/ nanotubes, plays an important role in controlling the physicochemical behavior of the whole system. Weak photoresponse of TNTs before annealing which has been considered to be a result of amorphous structure of tube walls now can also be related to the lack of electrical and mechanical contact between nanotubes and the substrate. Annealing nanotubes in air transforms amorphous TiO 2 to crystalline polymorphs and forms a new oxide layer on the surface of titanium which noticeably changes the electrochemical and photo-electrochemical behavior of TNTs. Sintering of nanotubes bottom to the underlying oxide layer, formed during annealing, has found to be very important since a good electrical connection is necessary for charge carriers passage into the metal substrate. Crystalline structure of tube walls causes different formation, separation and traortation of charges in contrast to the as-formed nanotubes which is mainly diffusion controlled and is independent of electrode potential. Capacitance behavior of annealed nanotubes was explained using a qualitative model showing two kinds of behavior in the anodic and cathodic potential regions. In the anodic region capacitance of TNTs is mainly controlled by the capacitance of semiconductor space charge layer adjacent to the underlying substrate but in the cathode region capacitance is controlled by chemical capacitance of double layer along the internal surface area of nanotubes. In the cathodic region nanotubes behave as a super capacitor showing several order of magnitude higher values of capacitance. Nitrogen was successfully incorporated into the titania barrier layer structure during anodization of titanium in ammonium containing solutions. It has been shown that nitrogen incorporation is responsible for the increase of the visible light photoresponse of barrier layers due to the formation of electronic localized states within the energy gap of TiO 2 above the valence band edge. In spite of some weak visible photoresponse from TNTs formed in ammonium containing electrolytes and after annealing, it seems that the mechanism of charge separation, traort and collection for photocurrent generation within TNTs and nitrogen incorporated titania barrier layers are not coincide. According to the electronic band structure of doped titanium oxide and the model presented for photocurrent generation in TNTs it has been explained that doping if TNTs is not a powerful mean for extending their photoresponse to the visible region. Keywords : TiO 2 nanotubes, (Photo-) electrochemical, Annealing, Photocurrent, Adhesion, Re-anodization, Doping, Nitrogen.