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A pentapeptide, $Boc-Leu-Ac_8c-Ala-Leu-Ac_8c-OMe 1$, an octapeptide, $Boc-Leu-Ac_8c-Ala-Leu-Ac_8c-Ala-Leu-Ac_8c-OMe 2$ and a tripeptide, $Boc-Aib-Ac_8c-Aib-OMe 3$ containing the 1-aminocyclooctane-1-carboxylic acid residue $(Ac_8c)$ were synthesized and conformationally characterized by x-ray diffraction studies in the crystal state. Peptides 1 and 2 were also studied by NMR in $CDCl_3$ solution. Peptide 1 adopts a purely $3_{10}-helical$ conformation in crystals, stabilized by three intramolecular 1 \longleftarrow 4 hydrogen bonds. Peptide 2 in crystals is largely $3_{10}-helical$ with distortion in the backbone at the N-terminus by the insertion of a water molecule between $Ac_8c$ (2) CO and Ala (6) NH groups. Peptide 3 forms a $C_{10}-ring$ structure, i.e. a type III (III) \beta - turn conformation stabilized by an intramolecular 1 \longleftarrow 4 hydrogen bond. Five cyclooctane rings assume boat-chair conformations, whereas the sixth $[Ac_8c(8) in 2]$ is appreciably distorted, resembling a chiral intermediate in the pseudorotational pathway from the boat-chair to the twisted boat-chair conformation. Internal bond angles of the cyclooctane rings are appreciably distorted from the tetrahedral value, a characteristic feature of the cyclooctane ring. Peptide 1 crystallized in the space group $P2_12_12_1$ with a = 11.900(4) A, b = 18.728(6) A, c = 20.471(3) A and Z = 4. The final $R_1$ and $wR_2$ values are 0.0753 and 0.2107, respectively, for 3901 observed reflections $[F_o \geq \sigma 3(F_o)]$. Peptide 2 crystallized in space group $P2_1$ with a = 12.961(5) A, b = 17.710(10) A, c = 15.101(7) A, = 108.45(4)A and Z = 2. The final $R_1$ and $wR_2$ values are 0.0906 and 0.1832, respectively, for 2743 observed reflections $[F_o \geq \sigma 3(F_o)]$. $^1H-NMR$ studies on both the peptides strongly suggest the persistence of $3_{10}-helical$ conformations in solution. Peptide 3 crystallized in the space group $P2_1/n$, with a = 10.018(1) A, b = 20.725(1) A, c = 12.915(1) A and Z = 4. The final $R_1$ and $wR_2$ values are 0.0411 and 0.1105, respectively, for 3634 observed reflections $[F_o \geq \sigma 4(F_o)]$.